kconfig: nconf: rewrite help texts

>From: Roland Eggner <edvx1@systemanalysen.net>

Rewrite all help texts.  During several years lazy (incomplete) updates have
left behind a rather thick layer of dust.  Intentions:
(1)  Global help called by <F1> should document all  _currently_  implemented
     keybindings.
(2)  Different help texts called by <F3> resp. <F8><F1> should be consistent
     with (1) and with implementation:
     •  on plain menu entry
     •  in radiolist window
     •  in input windows for text, decimal or hexadecimal values
     •  in filename selection windows <F6> <F7>
     •  SymSearch specific help called by <F8> followed by <F1>
(3)  More reasonable window titles:
     Rename window title            s/README/Global help/
     Rename variable    s/nconf_readme/nconf_global_help/
     Rename window title       s/Instructions/Short help/
(4)  Consider which hints are most useful for first-time-users.
Signed-off-by: Roland Eggner <edvx1@systemanalysen.net>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[yann.morin.1998@free.fr: a few additional fixes]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>