Update doctrings for continuous and categorical covariate docstrings

[watiss]

There were sources of confusion among more than one users about how the model consumes this, so this is adding some more context to the docstrings to provide clarity on that.

[codecov[bot]]

Codecov Report

Merging #1637 (9a0da1f) into master (c6b2b12) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1637   +/-   ##
=======================================
  Coverage   91.26%   91.26%           
=======================================
  Files         116      116           
  Lines        9193     9193           
=======================================
  Hits         8390     8390           
  Misses        803      803           

Impacted Files	Coverage Δ
scvi/utils/_docstrings.py	100.00% <ø> (ø)

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[adamgayoso]

meeseeksdev backport to 0.17.x