clarify Note [Preproccesing invocations]

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

This is just a documentation change; a question came up in the Matrix room as to what the observation about C comments meant in Note [Preprocessing invocations].

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][notes] and cross-reference it from the relevant places.
• add a [testcase to the testsuite][adding test]. (n/a)
• updates the users guide if applicable (n/a)
• mentions new features in the release notes for the next release (n/a)