fix ambiguity in `const` documentation

Where is the key part of this patch? That is, what should reviewers look at first? There is only one changed line in the documentation of const. The reasons are discussed in the related issue.

Fixes #20412 (closed).

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 and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a Note 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

Most of the points above don't apply to this PR, as far as I understand. Checking them doesn't necessarily mean that they have been done/applied. It could be that they seemed inapplicable.