Help: Fix inaccuracies in INSTALL_REMOVE_ENVIRONMENT_RPATH docs
The previous docs added by !3544 (merged) assumed one had to set
INSTALL_RPATH for this target property to have an effect, but the actual condition is only that the install rpath is different to the build rpath. Even if the install rpath is empty, it is possible that CMake will rewrite the rpath during install because the build rpath could be non-empty.
This MR also removes a stray indent from the generated
file(RPATH_CHANGE) command, which was also introduced by !3544 (merged). This makes this MR not strictly a doc-only change, but it is only a trivial code change.