Add \s for 'strong' text, i.e. bold rather than italics. I've missed
this a couple of times in Halibut markup recently (in particular, it's
handy to have a typographical distinction between 'this term is
emphasised because it's new' and 'this term is emphasised because I
want you to pay attention to it'), so here's an implementation,
basically parallel to \e.

One slight oddity is that strong text in headings will not be
distinguished in some output formats, since they already use bolded
text for their headings.


git-svn-id: svn://svn.tartarus.org/sgt/halibut@9772 cda61777-01e9-0310-a592-d414129be87e
Index term tweaks in manual for \s to quell warnings when building, etc.


git-svn-id: svn://svn.tartarus.org/sgt/halibut@9774 cda61777-01e9-0310-a592-d414129be87e