Tidy up cwd/home mount explanation, from sylabs198

This fixes a subpart of https://github.com/apptainer/apptainer-userdocs/issues/239 by merging

https://github.com/sylabs/singularity-userdocs/pull/198 which fixed
https://github.com/sylabs/singularity-userdocs/issues/189

The original PR description was:

Note the situation in which CWD is not mounted, how to avoid mounting $HOME if CWD is under $HOME etc. Correct /clarify some odd stuff around --contain-all Fixes https://github.com/sylabs/singularity-userdocs/issues/189

apptainer / apptainer-userdocs

Tidy up cwd/home mount explanation, from sylabs198 #251