Update _base.py

[pgierz]

Adds an additional note to clarify get_xticklabels

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@68d5e2f). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master       #1   +/-   ##
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  Coverage          ?   74.34%           
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  Files             ?      389           
  Lines             ?    90350           
  Branches          ?     9707           
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  Hits              ?    67171           
  Misses            ?    20647           
  Partials          ?     2532

Impacted Files	Coverage Δ
lib/matplotlib/axes/_base.py	78.35% <ø> (ø)

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