doc: warn user of ~ usage

Using ~ inside path variables (like LDP_PATH) will be expanded as the
root user, rather than the user who set the variable. This causes ~ to
refer to the root user's home directory, which is likely not the end
user's intention.

Provide a note on this behavior so end user can avoid using ~.

Signed-off-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Liangxing Wang <liangxing.wang@arm.com>
Change-Id: I57ca74edbbe94695f30c046e005e607fa1a3c3d4