btrfs-progs: docs: update system limits texts in sect. 5

Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/btrfs-man5.asciidoc

@@ -641,10 +641,11 @@ maximum file name length::
 
 maximum symlink target length::
 depends on the 'nodesize' value, for 4k it's 3949 bytes, for larger nodesize
-it's 4095
+it's 4095 due to the system limit PATH_MAX
 +
-The symlink target may not be a valid path, ie the path name components
-can exceed the limits, there's no content validation at `symlink`(3) creation.
+The symlink target may not be a valid path, ie. the path name components
+can exceed the limits (NAME_MAX), there's no content validation at `symlink`(3)
+creation.
 
 maximum number of inodes::
 2^64^ but depends on the available metadata space as the inodes are created
@@ -657,9 +658,9 @@ maximum file length::
 inherent limit of btrfs is 2^64^ (16 EiB) but the linux VFS limit is 2^63^ (8 EiB)
 
 maximum number of subvolumes::
-2^64^ but depends on the available metadata space, the space consumed by all
-subvolume metadata includes bookkeeping of the shared extents can be large (MiB,
-GiB)
+the subvolume ids can go up to 2^64^ but the number of actual subvolumes
+depends on the available metadata space, the space consumed by all subvolume
+metadata includes bookkeeping of shared extents can be large (MiB, GiB)
 
 maximum number of hardlinks of a file in a directory::
 65536 when the `extref` feature is turned on during mkfs (default), roughly
@@ -727,6 +728,7 @@ When set on a directory, all newly created files will inherit this attribute.
 No other attributes are supported.  For the complete list please refer to the
 `chattr`(1) manual page.
 
+
 CONTROL DEVICE
 --------------
 
@@ -758,6 +760,7 @@ The control device is not strictly required but the device scanning will not
 work and a workaround would need to be used to mount a multi-device filesystem.
 The mount option 'device' can trigger the device scanning during mount.
 
+
 SEE ALSO
 --------
 `acl`(5),