Improve accuracy of apropos commands that search doc strings

Improve accuracy of apropos commands that search doc strings

It is conceptually wrong for apropos commands that search doc
strings to look for matches of several words only on the same
line, because division of doc strings between lines is
ephemeral.
* lisp/apropos.el (apropos-parse-pattern): Accept an optional
argument MULTILINE-P, and if that is non-nil, produce regexps that
match words in the list even if they are separated by line
boundaries.
(apropos-value, apropos-local-value, apropos-documentation): Use
the new optional argument in apropos commands that search
multiline text, such as doc strings.

* src/search.c (Fposix_looking_at, Fposix_string_match)
(Fposix_search_backward, Fposix_search_forward): Make sure Posix
appears in the doc strings near REGEXP, for better matches.