flags: clarify documentation that LocalFlags related function do not modify the state

[niamster]

This addresses #1825

A user might confuse LocalFlags (or InheritedFlags) as a way to set local command flags in the same way as it is done via Flags (or PersistentFlags).
It appears that the purpose ofLocalFlags is to return a FlagSet for further examination.
Technically it's possible to update some flags returned by LocalFlags as FlagSet references original flags using pointers.

This PR attempts to clarify the documentation.
Also this PR removes lflags and iflags from the Command structure to avoid confusion as they are not used beyond the scope of LocalFlags or InheritedFlags methods. Just recreate the instance on every call to relevant function.

NOTE: it's not possible to make LocalFlags return a value (rather than a pointer) as it breaks some APIs.

NOTE: local tests pass

>make all > /dev/null 2>&1 && echo OK
OK

[CLAassistant]

All committers have signed the CLA.

[marckhouzam]

Thanks!