flag: document purpose of FlagSet.Name

[mark-rushakoff]

The godoc for NewFlagSet says

NewFlagSet returns a new, empty flag set with the specified name and error handling property.

and (*FlagSet).Name() says

Name returns the name of the flag set.

But it's unclear to me why a flag set has a name at all. Skimming through the source code, it looks like the name field is only used in (*FlagSet).defaultUsage().

The intent of the name would be more clear if NewFlagSet's godoc mentioned that the name will be used in the default usage, if you do not override the Usage field.

[gopherbot]

Change https://golang.org/cl/102955 mentions this issue: flag: document use of FlagSet name parameter