Remove convention sections

[ssiccha]

• Remove "Conventions" sections from the docs
  • We don't have any conventions. That we only handle finite fields should rather go into the introduction.
• Fix a typo
• Document that we can only handle finite fields

[codecov[bot]]

Codecov Report

Merging #273 (4930846) into master (36e0679) will increase coverage by 0.00%. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #273   +/-   ##
=======================================
  Coverage   78.00%   78.00%           
=======================================
  Files          39       39           
  Lines       17480    17480           
=======================================
+ Hits        13635    13636    +1     
+ Misses       3845     3844    -1     

Impacted Files	Coverage Δ
gap/base/recognition.gd	100.00% <ø> (ø)
gap/perm/giant.gi	90.09% <0.00%> (-0.68%)	:arrow_down:
gap/projective/d247.gi	76.35% <0.00%> (+1.27%)	:arrow_up:

[fingolfin]

I would actually like to change that we only handle finite fields, and e.g. also handle Z/nZ...