encoding/csv: clarify that this package supports RFC 4180

The intent of this comment is to reduce the number of issues opened
against the package to add support for new kinds of CSV formats, such as
issues #3150, #8458, #12372, #12755.

Change-Id: I452c0b748e4ca9ebde3e6cea188bf7774372148e
Reviewed-on: https://go-review.googlesource.com/23401Reviewed-by: Andrew Gerrand <adg@golang.org>

src/encoding/csv/reader.go

@@ -3,6 +3,8 @@
 // license that can be found in the LICENSE file.
 
 // Package csv reads and writes comma-separated values (CSV) files.
+// There are many kinds of CSV files; this package supports the format
+// described in RFC 4180.
 //
 // A csv file contains zero or more records of one or more fields per record.
 // Each record is separated by the newline character. The final record may