Fix #22300 Document GHC's extensions to valid whitespace

As suggested in #22300 (closed), this proposes extending the GHC User's Guide to document GHC's extensions to valid whitespace.

Please take a few moments to verify that your commits fulfill the following:

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to [Notes][notes] and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a [Note][notes] and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• if your MR affects library interfaces (e.g. changes base) or affects whether GHC will accept user-written code, please add the ~"user facing" label.
• updates the users guide if applicable
• mentions new features in the release notes for the next release