doc: expand on 'N' command, mention \s,\S,\<,\>

[Assaf Gordon]

Hello,

Attached two small patches to extend the documentation.
I'm not sure if \s,\S,\<,\> where always supported and undocumented,
but with addition of the DFA engine, they will always be supported now.

Comments welcomed,
 - assaf

P.S.
When rebuilding the docs, I noticed to issues:
1. parallel make (make -j) fails, seems like the man-page does not directly 
depend on the 'sed/sed' binary, and so it runs before the compilation completes.

2. When updating 'doc/sed-in.texi', running 'make doc/sed.html' once only 
regenerates 'doc/sed.texi',
and a second invocation is required to actually regenerate 'doc/sed.html'.

I'll try to fix these soon.

doc-2016-08-09.patch
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