Improve doc comments in promauto

In particular, point out that `With(nil)` is valid.

Signed-off-by: beorn7 <beorn@grafana.com>

prometheus/promauto/auto.go

@@ -127,29 +127,30 @@
 // separate package?
 //
 // The main problem is that registration may fail, e.g. if a metric inconsistent
-// with the newly to be registered one is already registered. Therefore, the
+// with or equal to the newly to be registered one is already registered.
-// Register method in the prometheus.Registerer interface returns an error, and
+// Therefore, the Register method in the prometheus.Registerer interface returns
-// the same is the case for the top-level prometheus.Register function that
+// an error, and the same is the case for the top-level prometheus.Register
-// registers with the global registry. The prometheus package also provides
+// function that registers with the global registry. The prometheus package also
-// MustRegister versions for both. They panic if the registration fails, and
+// provides MustRegister versions for both. They panic if the registration
-// they clearly call this out by using the Must… idiom. Panicking is a bit
+// fails, and they clearly call this out by using the Must…  idiom. Panicking is
-// problematic here because it doesn't just happen on input provided by the
+// problematic in this case because it doesn't just happen on input provided by
-// caller that is invalid on its own. Things are a bit more subtle here: Metric
+// the caller that is invalid on its own. Things are a bit more subtle here:
-// creation and registration tend to be spread widely over the codebase. It can
+// Metric creation and registration tend to be spread widely over the
-// easily happen that an incompatible metric is added to an unrelated part of
+// codebase. It can easily happen that an incompatible metric is added to an
-// the code, and suddenly code that used to work perfectly fine starts to panic
+// unrelated part of the code, and suddenly code that used to work perfectly
-// (provided that the registration of the newly added metric happens before the
+// fine starts to panic (provided that the registration of the newly added
-// registration of the previously existing metric). This may come as an even
+// metric happens before the registration of the previously existing
-// bigger surprise with the global registry, where simply importing another
+// metric). This may come as an even bigger surprise with the global registry,
-// package can trigger a panic (if the newly imported package registers metrics
+// where simply importing another package can trigger a panic (if the newly
-// in its init function). At least, in the prometheus package, creation of
+// imported package registers metrics in its init function). At least, in the
-// metrics and other collectors is separate from registration. You first create
+// prometheus package, creation of metrics and other collectors is separate from
-// the metric, and then you decide explicitly if you want to register it with a
+// registration. You first create the metric, and then you decide explicitly if
-// local or the global registry, and if you want to handle the error or risk a
+// you want to register it with a local or the global registry, and if you want
-// panic. With the constructors in the promauto package, registration is
+// to handle the error or risk a panic. With the constructors in the promauto
-// automatic, and if it fails, it will always panic. Furthermore, the
+// package, registration is automatic, and if it fails, it will always
-// constructors will often be called in the var section of a file, which means
+// panic. Furthermore, the constructors will often be called in the var section
-// that panicking will happen as a side effect of merely importing a package.
+// of a file, which means that panicking will happen as a side effect of merely
+// importing a package.
 //
 // A separate package allows conservative users to entirely ignore it. And
 // whoever wants to use it, will do so explicitly, with an opportunity to read
@@ -252,7 +253,8 @@ type Factory struct {
 }
 
 // With creates a Factory using the provided Registerer for registration of the
-// created Collectors.
+// created Collectors. If the provided Registerer is nil, the returned Factory
+// creates Collectors that are not registered with any Registerer.
 func With(r prometheus.Registerer) Factory { return Factory{r} }
 
 // NewCounter works like the function of the same name in the prometheus package