docs: clarify global gitignore and fix typos

[LiviaMedeiros]

The Make sure your credentials won't be committed part of guide implies that ~/.gitignore_global is a default path for global gitignore.

If anyone followed this part too literally, I'd recommend to doublecheck their setup with something like:

$ cat "$(git config --global core.excludesFile || echo ${XDG_CONFIG_HOME:-${HOME}/.config}/git/ignore)"
# should show the contents of global gitignore

This is applicable to Linux; I'm unsure about other platforms.
git config --global core.excludesfile should work everywhere, but note that setting it overrides file in default location (if any).

[codecov]

Codecov Report

Base: 84.09% // Head: 84.09% // No change to project coverage 👍

Coverage data is based on head (991ff27) compared to base (5247dc0).
Patch has no changes to coverable lines.

❗ Current head 991ff27 differs from pull request most recent head f5b9c76. Consider uploading reports for the commit f5b9c76 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #627   +/-   ##
=======================================
  Coverage   84.09%   84.09%           
=======================================
  Files          37       37           
  Lines        4074     4074           
=======================================
  Hits         3426     3426           
  Misses        648      648           

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