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.