[clang][driver] Restore the original help text for `-I`

The help text for `-I` was recently expanded in [1]. The expanded
version focuses on explaining the semantics of `-I` in Clang. We are now
in the process of adding support for `-I` in Flang and this new
description is incompatible with the semantics of `-I` in Flang. This
was brought up in this review:
  * https://reviews.llvm.org/D93453

This patch reverts the original change in Options.td. This way the help
text for `-I` remains generic enough so that it applies to both Clang
and Flang.

The expanded description of `-I` from [1] is moved to the
`DocBrief` field for `-I`. This field is prioritised over the help text
when generating ClangCommandLineReference.rst, so the user facing
documentation for Clang retains the expanded description:
  * https://clang.llvm.org/docs/ClangCommandLineReference.html
`DocBrief` fields are currently not used in Flang.

As requested in the reviews, the help text and the expanded description
are slightly refined.

[1] Commit: 8dd4e3ce

Differential Revision: https://reviews.llvm.org/D94169