Update wording to reflect rule severity

[sharwell]

All rules are warnings, so the term 'should' is used throughout.

Fixes #2036
Fixes #2272

📝 I didn't rename all the types that have Must in the name of the type.

[codecov]

Codecov Report

Merging #2459 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2459   +/-   ##
=======================================
  Coverage   97.28%   97.28%           
=======================================
  Files         734      734           
  Lines       94977    94977           
  Branches     3470     3470           
=======================================
  Hits        92396    92396           
  Misses       1741     1741           
  Partials      840      840

Impacted Files	Coverage Δ
...irectivesMustBeOrderedAlphabeticallyByNamespace.cs	100% <ø> (ø)	⬆️
...taticReadonlyFieldsMustBeginWithUpperCaseLetter.cs	69.23% <ø> (ø)	⬆️
...A1005SingleLineCommentsMustBeginWithSingleSpace.cs	100% <ø> (ø)	⬆️
...A1025CodeMustNotContainMultipleWhitespaceInARow.cs	94.82% <ø> (ø)	⬆️
...arameterDocumentationMustMatchElementParameters.cs	92.3% <ø> (ø)	⬆️
...Cop.Analyzers.Test/SpacingRules/SA1024UnitTests.cs	100% <ø> (ø)	⬆️
...ringRules/SA1207ProtectedMustComeBeforeInternal.cs	90% <ø> (ø)	⬆️
...Rules/SA1604ElementDocumentationMustHaveSummary.cs	96.66% <ø> (ø)	⬆️
...ivateReadonlyFieldsMustBeginWithUpperCaseLetter.cs	70% <ø> (ø)	⬆️
...A1650ElementDocumentationMustBeSpelledCorrectly.cs	100% <ø> (ø)	⬆️
... and 121 more

---

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 002898b...0878d9b. Read the comment docs.