Document behavior of backticks in usage string

[AlexNPavel]

Pflag changes the type of a flag to match a string in back quotes in the usage string. For example, if flag "test" had usage message "this sets up the testtype variable", the output would be:

      --test testtype  this sets up the testtype variable

This is unexpected behavior and is also not documented anywhere except in this godoc for the UnquoteUsage function: https://godoc.org/github.com/spf13/pflag#UnquoteUsage

This should be documented somewhere more visibly as back quotes are commonly used in descriptions and usage text.

[dpopp07]

I would love if this behavior could be customized or turned off. I can't always control the usage messages in my CLI (long story short) and they often have back quotes that are not intended to be used this way. It's causing some confusion and it's a bummer that I can't just default to the "educated guess of the type of the flag's value".

I looked through the code and don't see an easy or obvious way to implement that so all of to this to say:

I'm offering a +1 for this being documented. At the very least, users should know that it's going to happen.

[IzhakJakov]

👍