DOC: Spellcheck of merging.rst, reshaping.rst and timeseries.rst

[tommyod]

This PR is a continuation of my reading through the docs, see #19017, #18973, #18948, #18941.

The changes are:

• In merging.rst, reshaping.rst and timeseries.rst:
  • Function references as links
  • Backticks `` around Series, DataFrame
  • Minor rephrasing of sentences, spelling, etc
• Replaced some instances of python with Python and numpy with NumPy globally in *.rst files. I hope to add a lint rule in a future PR.

Comments/reviews are very welcome.

[codecov]

Codecov Report

Merging #19081 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #19081      +/-   ##
==========================================
- Coverage   91.54%   91.51%   -0.03%     
==========================================
  Files         148      148              
  Lines       48804    48804              
==========================================
- Hits        44676    44664      -12     
- Misses       4128     4140      +12

Flag	Coverage Δ
#multiple	89.89% <ø> (-0.03%)	⬇️
#single	41.63% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/plotting/_converter.py	65.22% <0%> (-1.74%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4a8496b...4f1a867. Read the comment docs.

[jreback]

thanks @tommyod keep em coming!

be sure to glance at the built docs to make sure the edits are ok: http://pandas-docs.github.io/pandas-docs-travis/

built by the CI so can take some time