Clarify documentation of functions reading character events

Clarify documentation of functions reading character events

* doc/lispref/help.texi (Describing Characters):
* doc/lispref/commands.texi (Keyboard Events)
(Reading One Event, Classifying Events): Make the distinction
between characters and character events more explicit.

* src/keymap.c (Ftext_char_description)
(Fsingle_key_description):
* src/lread.c (Fread_char, Fread_char_exclusive): Doc fixes,
to make a clear distinction between a character input event
and a character code.  (Bug#32562)