base: Improve String & IsString documentation

Where is the key part of this patch? That is, what should reviewers look at first?

IsString & String docs

user-facing Tracking: #17929

Please take a few moments to address the following points:

• if your MR may break existing programs (e.g. touches base or causes the compiler to reject programs), please describe the expected breakage and add the ~"user facing" label. This will run ghc/head.hackage> to characterise the effect of your change on Hackage.
• ensure that your commits are either individually buildable or squashed
• ensure that your commit messages describe what they do (referring to 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.
• [ ] updates the users guide if applicable
• [ ] mentions new features in the release notes for the next release