doc-rst: parse-headers: improve delimiters to detect symbols

As we had to escape the symbols for the ReST markup to not do
the wrong thing, the logic to discover start/end of strings
are not trivial. Improve the end delimiter detection, in order
to highlight more occurrences of the strings.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>