fmt: more doc needed for Formatter

[dolmen]

Current (go1.15, tip) documentation of interface fmt.Formatter:

Formatter is implemented by any value that has a Format method. The implementation controls how State and rune are interpreted, and may call Sprint(f) or Fprint(f) etc. to generate its output.

This is insufficient to tell how one must implement this interface:

• which verbs should be handled for each type?
• how to handle unhandled verbs?
• how to forward unhandled verbs to Printf?
• how to handle unhandled width?
• how to handle unhandled precision?
• lack of example

[martisch]

There are no constraints how to implement formatters apart from the function signature. Its up to the developer to decide what should be handled how for each type. Which verbs produce output and which dont. Which verbs are implemented directly and which are formatted using other functions.

An example could be added. Contributions welcome.

[dolmen]

Yes, there are no constraints. But guidelines or best practices would still be helpful.

Also the stdlib has very few implementation of that interface (grep 'Format(fmt\.State,' $(go env GOROOT)/api/*.txt lists only 2 in package math/big) so reference code is limited.