Help: Clarify add_custom_command DEPENDS conversion to file paths

In commit f5126badd8 (add_custom_command: convert DEPENDS path arguments
to absolute paths, 2019-12-18, v3.17.0-rc1~263^2) we updated the
documentation to describe the behavior introduced by that commit.
However, the behavior was removed again by commit fd0ba705ce
(add_custom_command: check if a relative path should be an in-source
path, 2020-01-09, v3.17.0-rc1~141^2~4) without updating the
documentation.

Update the documentation again to describe the behavior as of the latter
commit.  Spell out the steps that `cmLocalGenerator::GetRealDependency`
and `cmTargetTraceDependencies::IsUtility` use to add file-level and
target-level dependencies.

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