Documentation: Protect file extensions from start of line.

Documentation: Protect file extensions from start of line.

In the documentation files, we use file extensions as words to simplify
the text. When compiling man pages from the Markdown files, it is
possible that an extension lands at the beginning of a line.

In such a case, the extension is mistakenly identified as a Groff macro.
It seems as though Groff simply ignores it since it is not a known
macro, but emits a warning about the syntax.

This was caught thanks to the debian package's lintian output at:

http://lintian.debian.org/maintainer/jmtd@debian.org.html#bup

Since we're putting highlighting on file extensions, we should add it to
all cases, even though there's not risk of it landing at the beginning
of a line. This way, the documentation looks better standardized.

Signed-off-by: Gabriel Filion <lelutin@gmail.com>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>