Add comment to LogFormatterParams struct's fields (#1711)

By https://github.com/gin-gonic/gin/issues/1701, I thought it's necessary.

logger.go

@@ -46,13 +46,22 @@ type LogFormatter func(params LogFormatterParams) string
 // LogFormatterParams is the structure any formatter will be handed when time to log comes
 type LogFormatterParams struct {
 	Request *http.Request
+
+	// TimeStamp shows the time after the server returns a response.
 	TimeStamp time.Time
+	// StatusCode is HTTP response code.
 	StatusCode int
+	// Latency is how much time the server cost to process a certain request.
 	Latency time.Duration
+	// ClientIP equals Context's ClientIP method.
 	ClientIP string
+	// Method is the HTTP method given to the request.
 	Method string
+	// Path is a path the client requests.
 	Path string
+	// ErrorMessage is set if error has occurred in processing the request.
 	ErrorMessage string
+	// IsTerm shows whether does gin's output descriptor refers to a terminal.
 	IsTerm bool
 }