Fix Pike Mode's autodoc doc comments style's continued lines.

Fix Pike Mode's autodoc doc comments style's continued lines.

* lisp/progmodes/cc-engine.el (c-forward-sws, c-backward-sws): Recognize
matches of c-doc-line-join-re as syntactic whitespace.
(c-find-decl-prefix-search): Recognize and move over matches of
c-doc-line-join-re as whitespace.
(c-find-decl-spots): Before moving backward a char, check (bobp).  Before
moving forward over a comment, check it isn't possibly a "bright" comment.

* lisp/progmodes/cc-fonts.el (c-get-doc-comment-style): New function,
extracted from c-compose-keywords-list.
(c-compose-keywords-list): Call the above new function.
(pike-font-lock-keywords, pike-font-lock-keywords-2)
(pike-font-lock-keywords-3): Call c-set-doc-comment-res.
(c-doc-line-join-re, c-doc-bright-comment-start-re, c-doc-line-join-end-ch):
New variables.
(c-set-doc-comment-re-element, c-set-doc-comment-char-list): New macros.
(c-set-doc-comment-res): New function.
(c-font-lock-doc-comments): For consistency and repeatability, in a sequence
of C++ style doc comments, don't fontify the region between BOL and the
comment marker.
(autodoc-line-join-re, autodoc-bright-comment-start-re)
(autodoc-line-join-end-ch): New variables.

* lisp/progmodes/cc-mode.el (c-doc-fl-decl-start, c-doc-fl-decl-end): New
functions.
(c-change-expand-fl-region, c-context-expand-fl-region): Call the above two
new functions for extra possibilities for the start and end of a construct.

* doc/misc/cc-mode.texi (Doc Comments): Add a sentence drawing attention to
the possibility of fontifying constructs within a doc comment.