Help: Clarify and improve readability of link-related file subcommands

This MR aims to make the docs for the file(READ_SYMLINK) and file(CREATE_LINK) subcommands read more naturally. The docs were also a bit narrow in assuming links only operated on files, but they could also be used for directories. I've renamed the command arguments and reworked the text a bit to help address this particular point.

changed milestone to %3.14.0

added area:doc label

Warnings:

• Eligible for the 3.14.x branch.

The warnings do not need to be fixed, but it is recommended to do so.

Cc: @kyle.edwards @tusharpm

I'm not crazy about the argument name <original>, but the usual terms for the equivalent unix command (ln) are "source" and "target", which would be inappropriate here in CMake's context since those already have other stronger meanings. The name can't imply a file or a directory, since it could be either. I previously had fileOrDir but that then made the text around hard links a bit cumbersome. Happy to consider other better suggestions.

I also expanded the docs for CREATE_LINK to mention a specific reason for COPY_ON_ERROR. That particular scenario was encountered by the nightly tests on some build machines, so it is worth highlighting.

The modification looks good to me. The note about COPY_ON_ERROR is useful.

Do: stage

added workflow:nightly-testing label

assigned to @brad.king

Do: merge

removed workflow:nightly-testing label

mentioned in commit 566652b0

merged

mentioned in commit 926a97e9