docs: mount.md - idmapped mounts only work for root user

As reported in #24918 and clarified in containers/crun#1632 (comment)

Signed-off-by: Evstifeev Roman <[email protected]>

docs/source/markdown/options/mount.md

@@ -31,7 +31,8 @@ Options specific to type=**volume**:
 - *U*, *chown*: *true* or *false* (default if unspecified: *false*). Recursively change the owner and group of the source volume based on the UID and GID of the container.
 
 - *idmap*: If specified, create an idmapped mount to the target user namespace in the container.
-  The idmap option supports a custom mapping that can be different than the user namespace used by the container.
+  podman must be launched as root to use this option. The idmap option supports a custom mapping that can be different than the user 
+  namespace used by the container.
   The mapping can be specified after the idmap option like: `idmap=uids=0-1-10#10-11-10;gids=0-100-10`.  For each triplet, the first value is the
   start of the backing file system IDs that are mapped to the second value on the host.  The length of this mapping is given in the third value.
   Multiple ranges are separated with #.  If the specified mapping is prepended with a '@' then the mapping is considered relative to the container