Correct nmod_poly example.

There is no nmod_set_coeff_si() function implemented.
nmod_poly: fix signatures of the sqrt functions in the documentation.
Merge remote-tracking branch 'upstream/trunk' into trunk
Fix markup in the documentation to be able to build the manual.
Merge branch 'trunk' of https://github.com/wbhart/flint2 into trunk

Conflicts:
	nmod_poly/doc/nmod_poly.txt