kernel-doc: limit the "section header:" detection to a select few

kernel-doc: limit the "section header:" detection to a select few

kernel-doc currently identifies anything matching "section header:"
(specifically a string of word characters and spaces followed by a
colon) as a new section in the documentation comment, and renders the
section header accordingly.

Unfortunately, this turns all uses of colon into sections, mostly
unintentionally. Considering the output, erroneously creating sections
when not intended is always worse than erroneously not creating sections
when intended. For example, a line with "http://example.com" turns into
a "http" heading followed by "//example.com" in normal text style, which
is quite ugly. OTOH, "WARNING: Beware of the Leopard" is just fine even
if "WARNING" does not turn into a heading.

It is virtually impossible to change all the kernel-doc comments, either
way. The compromise is to pick the most commonly used and depended on
section headers (with variants) and accept them as section headers.

The accepted section headers are, case insensitive:

 * description:
 * context:
 * return:
 * returns:

Additionally, case sensitive:

 * @return:

All of the above are commonly used in the kernel-doc comments, and will
result in worse output if not identified as section headers. Also,
kernel-doc already has some special handling for all of them, so there's
nothing particularly controversial in adding more special treatment for
them.

While at it, improve the whitespace handling surrounding section
names. Do not consider the whitespace as part of the name.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>