Better documentation of Polytope's type parameter

[Antoinemarteau]

I noticed that some polytope docstrings were wrong regarding the definition of the D parameter of Polytope{D} (it's not the embedding dimension).

So I improved the docs and made num_point_dims(::Polytope) abstract because the result depend on the polytope implementation.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.31%. Comparing base (fb5235b) to head (d201988).

Files with missing lines	Patch %	Lines
src/ReferenceFEs/ExtrusionPolytopes.jl	0.00%	1 Missing ⚠️
src/ReferenceFEs/Polytopes.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1194      +/-   ##
==========================================
- Coverage   89.31%   89.31%   -0.01%     
==========================================
  Files         212      212              
  Lines       27013    27014       +1     
==========================================
  Hits        24128    24128              
- Misses       2885     2886       +1     

Flag	Coverage Δ
drivers	0.00% <0.00%> (ø)
extensions	0.00% <0.00%> (ø)
tests	89.31% <0.00%> (-0.01%)	⬇️

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