man: specify that ProtectProc= does not work with root/cap_sys_ptrace

man: specify that ProtectProc= does not work with root/cap_sys_ptrace

When using hidepid=invisible on procfs, the kernel will check if the
gid of the process trying to access /proc is the same as the gid of
the process that mounted the /proc instance, or if it has the ptrace
capability:

https://github.com/torvalds/linux/blob/v5.10/fs/proc/base.c#L723
https://github.com/torvalds/linux/blob/v5.10/fs/proc/root.c#L155

Given we set up the /proc instance as root for system services,
The same restriction applies to CAP_SYS_PTRACE, if a process runs with
it then hidepid=invisible has no effect.

ProtectProc effectively can only be used with User= or DynamicUser=yes,
without CAP_SYS_PTRACE.
Update the documentation to explicitly state these limitations.

Fixes #18997