bug#10885: Replace expressions: enhance functionality when searching in filled paragraphs

[Juri Linkov]

> The doc and manual would need updating; since at present
> search-whitespace-regexp is only documented to affect incremental
> searches.

There is one problem of using `search-whitespace-regexp'
in `query-replace-regexp'.

When the user doesn't want `SPC' to match any sequence of whitespace
characters in regexp isearch `C-M-s', it it possible to type `C-q SPC'.

But how to do the same in `query-replace-regexp'?

`C-M-% replace C-q SPC this' doesn't quote SPC as a literal character.