docs: indicate that tab_spanner() allows for use of units notation in its label= arg

[rich-iannone]

This is a simple documentation improvement for the tab_spanner() method. The cols_label() allows for the use of units notation syntax and has documentation to explain that. This was missing for tab_spanner() though it has equivalent functionality. This PR explains that units notation can be used in tab_spanner()

Fixes: #395

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.68%. Comparing base (0422164) to head (37772be).
Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #426      +/-   ##
==========================================
+ Coverage   86.37%   86.68%   +0.31%     
==========================================
  Files          42       42              
  Lines        4681     4685       +4     
==========================================
+ Hits         4043     4061      +18     
+ Misses        638      624      -14     

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[machow]

LGTM, thanks! (from pairing, we'll add a guide page for better explaining text handling / units #439)