bug#15166: 24.3.50; Isearch for an octal code

[Juri Linkov]

>> BTW, while looking at `isearch-quote-char' I noticed that it
>> requires a comment saying that `C-q SPC' is an obsolete feature
>> (that is now replaced by `M-s SPC').
>
> Why is C-q SPC obsolete?

The problem is that C-q SPC has limited applicability -
it works only in regexp isearch, and has no effect in
normal non-regexp isearch.  This might be too confusing
to users when typing the same key sequence unexpectedly
works differently in different isearch modes.

Initially C-q SPC was intended only for regexp isearch,
but after introduction of isearch-lax-whitespace mode
it can't be used in non-regexp mode because the C-q SPC
feature was based on regexps.

I don't propose to remove it now, keeping it for users
that rely on it in regexp search.  But to remove mentions
from documentation to not confuse users about the feature
that doesn't work in non-regexp mode.