DOC: Improve the docstring of Timedelta.delta redux

[chalmerlowe]

• closes #xxxx
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

rather than trying to pick and choose items from my previous PR #21135 (which somehow had a number of other items sucked into it) ...
gonna start from scratch here and close out the old PR.

tried to incorporate changes noted in previous conversations with @WillAyd and @jreback (thanks for the guidance!)

################################################################################
###################### Docstring (pandas.Timedelta.delta) ######################
################################################################################

Return the timedelta in nanoseconds (ns), for internal compatibility.

Returns
-------
int
    Timedelta in nanoseconds.

Examples
--------
>>> td = pd.Timedelta('1 days 42 ns')
>>> td.delta
86400000000042

>>> td = pd.Timedelta('3 s')
>>> td.delta
3000000000

>>> td = pd.Timedelta('3 ms 5 us')
>>> td.delta
3005000

>>> td = pd.Timedelta(42, unit='ns')
>>> td.delta
42

################################################################################
################################## Validation ##################################
################################################################################

[codecov]

Codecov Report

Merging #21138 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #21138   +/-   ##
=======================================
  Coverage   91.84%   91.84%           
=======================================
  Files         153      153           
  Lines       49499    49499           
=======================================
  Hits        45460    45460           
  Misses       4039     4039

Flag	Coverage Δ
#multiple	90.23% <ø> (ø)	⬆️
#single	41.88% <ø> (ø)	⬆️

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[jreback]

thanks!

[jreback]

thanks!