housekeeping: Improve Custom Formatters Docs

[worldbeater]

What kind of change does this PR introduce?

This PR documents custom validation text formatters. We describe how to use SingleLineFormatter with other separator characters, and also we describe how to override the default IValidationTextFormatter<string> used in ReactiveUI.Validation.

What is the current behavior?

Currently, the way to override the default IValidationTextFormatter<string> isn't documented.

What is the new behavior?

Now, the way to override the default IValidationTextFormatter<string> is documented.

What might this PR break?

Nothing.

[codecov]

Codecov Report

Merging #153 into main will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #153   +/-   ##
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  Coverage   62.14%   62.14%           
=======================================
  Files          16       16           
  Lines         906      906           
=======================================
  Hits          563      563           
  Misses        343      343           

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