Added clarity and doc of snapshot -i option

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Was:

snapshot [-r] <source> <dest>|[<dest>/]<name>

Create a writable/readonly snapshot of the subvolume <source> with the name
<name> in the <dest> directory.

If only <dest> is given, the subvolume will be named the basename of <source>.
If <source> is not a subvolume, btrfs returns an error. If -r is given, the snapshot will be readonly.

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Is:

snapshot [-r] [-i ] <subvolume> { <subdir>/<name> | <subdir> }

Create a snapshot of a <subvolume>. Call it <name> and place it in the <subdir>.

When only <subdir> is given, the subvolume will be named using the basename of <subvolume>.

(<subvolume> looks like a sub-directory, but is actually a btrfs subvolume rather than a subdirectory.)

-r
Make the new snapshot readonly.

-i <qgroupid>
Add the new snapshot to a qgroup (quota group). This option can be given multiple times.

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RATIONALE

1. First I think there's no reason why the -r option usage should be formatted any differently here than for the rest of this documentation. All other options above this are split off and in lines of their own. This is the only option that is spelled out in the text.

2. Also -i documentation is added. If you run $> btrfs subvolume snapshot you get:

btrfs subvolume snapshot: too few arguments
usage: btrfs subvolume snapshot [-r] [-i <qgroupid>] <source> <dest>|[<dest>/]<name>

Create a snapshot of the subvolume

Create a writable/readonly snapshot of the subvolume \<source> with
the name \<name> in the \<dest> directory.  If only \<dest> is given,
the subvolume will be named the basename of \<source>.

-r             create a readonly snapshot
-i \<qgroupid>  add the newly created snapshot to a qgroup. This
               option can be given multiple times.

3. The | and [...] leave me to wonder which happens first. Synopsis is clarified in standard notation.

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4. For someone learning this I think using the more exact term "subvolume" is more clear than simply "source". Furthermore, this is in agreement with the existing usage: on line 641 in cmds-subvolume.c which says this, "Create a snapshot of the subvolume, ..."

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5. Likewise "subdir" is more clear than "dest". (Check me out, but I don't think it's possible for dest to be anything other than a subdir.) Again in cmds-subvolume.c it says, " directory". Also a snapshot can't be placed in the root directory, so it has to be in a sub-directory.