bug#36767: 26.1; request: add more quick keys to the *Help* buffer

[Juri Linkov]

> What does windmove have to do with this?

The same argument:

>> Maybe describe-* commands could accept the same prefix arg
>> where C-u will reverse the value of help-window-select
>> the same way as C-u of windmove-display-* reverses the value
>> of windmove-display-no-select.

> That doesn't work well with cross-links in the doc strings, because
> you need to type the name of the symbol.  I want a command that will
> do the equivalent of TAB in the Help buffer, but which will work from
> another buffer.

This is easy to implement the same way as windmove-display-* implemented,
i.e. a prefix key will allow to select another window, execute the
next command in another window, then select the old window back.

>> +*** New command 'help-customize' ('c').
>> +This command will customize the variable or the face (if any).
>
> I think this should make it clear the variable/face are those whose
> doc string is being shown in *Help*.

Done.