Add CharacteristicSubgroups to the reference manual

[fingolfin]

Also fix typo in an example on its manpage, and add another example to it and
to NormalSubgroups which clearly shows the difference.

@hulpke added CharacteristicSubgroups, so it would be good to know if he is comfortable with making this attribute "official".

[hulpke]

Yes, these are better examples

[codecov]

Codecov Report

Merging #3529 into master will decrease coverage by 5.43%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3529      +/-   ##
==========================================
- Coverage   85.46%   80.03%   -5.44%     
==========================================
  Files         696      692       -4     
  Lines      344989   342192    -2797     
==========================================
- Hits       294845   273872   -20973     
- Misses      50144    68320   +18176

Impacted Files	Coverage Δ
lib/grp.gd	100% <ø> (ø)	⬆️
lib/ctbllatt.gi	4.94% <0%> (-79.62%)	⬇️
lib/proto.gi	13.4% <0%> (-71.14%)	⬇️
src/libgap-api.c	1.16% <0%> (-63.38%)	⬇️
lib/schursym.gi	10.6% <0%> (-58.51%)	⬇️
src/saveload.c	10.05% <0%> (-58.16%)	⬇️
lib/ctblpope.gi	19.61% <0%> (-57.87%)	⬇️
lib/addgphom.gi	8.69% <0%> (-56.53%)	⬇️
lib/ctblmaps.gi	20.68% <0%> (-55.8%)	⬇️
lib/teaching.g	33.26% <0%> (-54.82%)	⬇️
... and 198 more

[coveralls]

Coverage decreased (-0.0003%) to 85.289% when pulling 3658990 on fingolfin:mh/CharacteristicSubgroups into 384ccc4 on gap-system:master.