DOC: Updating str_pad docstring

[JesperDramsch]

• [x ] tests added / passed
• [ x] passes git diff upstream/master -u -- "*.py" | flake8 --diff

Pad documentation was describing a different behaviour, so I fixed it and added examples.

[codecov]

Codecov Report

Merging #22567 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #22567   +/-   ##
=======================================
  Coverage   92.04%   92.04%           
=======================================
  Files         169      169           
  Lines       50787    50787           
=======================================
  Hits        46745    46745           
  Misses       4042     4042

Flag	Coverage Δ
#multiple	90.45% <ø> (ø)	⬆️
#single	42.29% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/strings.py	98.63% <ø> (ø)	⬆️

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[JesperDramsch]

I messed up. It continues here: #22570