Fix missing description for ManyRelatedField and tested for SlugField

[StopMotionCuber]

This fixes #894

Here I went with a list description of "[$help_text]", which could be modified in case needed.

I have also adapted the tests based on that, they are passing with my changeset

[codecov]

Codecov Report

Base: 98.68% // Head: 98.71% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (393e157) compared to base (0307ff9).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #895      +/-   ##
==========================================
+ Coverage   98.68%   98.71%   +0.02%     
==========================================
  Files          68       68              
  Lines        7827     7921      +94     
==========================================
+ Hits         7724     7819      +95     
+ Misses        103      102       -1     

Impacted Files	Coverage Δ
drf_spectacular/openapi.py	97.57% <100.00%> (+<0.01%)	⬆️
tests/test_fields.py	100.00% <100.00%> (ø)
tests/test_command.py	100.00% <0.00%> (ø)
tests/test_warnings.py	100.00% <0.00%> (ø)
tests/test_postprocessing.py	100.00% <0.00%> (ø)
drf_spectacular/views.py	94.48% <0.00%> (+0.11%)	⬆️
drf_spectacular/plumbing.py	97.26% <0.00%> (+0.13%)	⬆️
drf_spectacular/drainage.py	99.20% <0.00%> (+0.23%)	⬆️
drf_spectacular/management/commands/spectacular.py	96.49% <0.00%> (+0.57%)	⬆️

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[StopMotionCuber]

Eh, appying this to my own API, it seems like the "[$help_text]" format is not pretty with Redoc. Possibly we should just take the description, as the type is already clear from the type field

[tfranzel]

Possibly we should just take the description, as the type is already clear from the type field

I agree

I will still need to think a bit about the implications before merging this.

[StopMotionCuber]

Then think 😄

As no tests were failing I'm assuming that this shouldn't break something.
My implementation is also making sure that the description is not overwritten in case it is already available. So, this would only lead to "more documentation", but not really to "changed documentation".

But take your time to evaluate whether there are really no unintended side-effect. After all it's you who needs to maintain the library