DOC: update Series.combine/DataFrame.combine doc strings

[topper-123]

This PR updates Series.combine/DataFrame.combine doc strings. In particular, it adds some short examples to the doc strings.

[codecov]

Codecov Report

Merging #18268 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #18268      +/-   ##
==========================================
- Coverage    91.4%   91.38%   -0.02%     
==========================================
  Files         164      164              
  Lines       49883    49878       -5     
==========================================
- Hits        45595    45581      -14     
- Misses       4288     4297       +9

Flag	Coverage Δ
#multiple	89.19% <ø> (-0.01%)	⬇️
#single	39.41% <ø> (-0.07%)	⬇️

Impacted Files	Coverage Δ
pandas/core/series.py	95% <ø> (ø)	⬆️
pandas/core/frame.py	97.8% <ø> (-0.1%)	⬇️
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/indexes/multi.py	96.38% <0%> (-0.02%)	⬇️

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[jreback]

thanks!