Touch up text around require statements w.r.t. -I / -L flags

[bradcray]

[based on feedback and suggestions by @psath]

Based on feedback in #24963, this PR strives to improve the documentation of the require statement to clarify that it doesn't accept general chpl command-line flags and refactors the description of the -I and -L flags to try and clarify their role in common to listing files on the command-line or require statements. While here, I also did some wordsmithing and formatting adjustments.

Resolves #24963.