feat(eslint-plugin): [naming-convention] better error message and docs for prefix/suffix

[villelahdenvuo]

• Make error message more clear about trimming.
• Docs: add a note about prefix being trimmed before format is validated to make it more noticeable.

Addresses #2187

Here's the current error message:

Proposed error message:

Variable name isStore trimmed as Store must match one of the following formats: UPPER_CASE, camelCase

Better wording can be considered, I tried to keep it short and informative.

[typescript-eslint]

Thanks for the PR, @villelahdenvuo!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[codecov]

Codecov Report

Merging #2195 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2195   +/-   ##
=======================================
  Coverage   93.42%   93.42%           
=======================================
  Files         173      173           
  Lines        7861     7862    +1     
  Branches     2242     2243    +1     
=======================================
+ Hits         7344     7345    +1     
  Misses        245      245           
  Partials      272      272           

Flag	Coverage Δ
#unittest	93.42% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
...kages/eslint-plugin/src/rules/naming-convention.ts	88.82% <100.00%> (+0.03%)	⬆️

[bradzacher]

thanks heaps for contributing to the clarity of the complicated rule!!