Clarify splitting/passing behaviour for --*-option(s) arguments

[tomsmeding]

Spawned by a question on IRC of what exactly --ghc-option, --ghc-options and --repl-options take as their input format, I reformulated and expanded on the relevant documentation. Correctness of this information was checked in two ways:

• Limited experimentation with the cabal CLI;
• Asking questions to @merijn on IRC (later in the same log linked above).

This list conspicuously excludes "looking at the code". My apologies.

Checklist for template B (does not modify behaviour or interface):

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).