DOC: stick-table: add missing documentation about gpt0 stored type

The store type 'gpt0' was present in code but was not documented.

The patch fix this and should be backported since 'gpt0' is supported.
[wt: ~1.6-dev4 hence all stable]

(cherry picked from commit 1a6b7254de849765bef34f7b65b92ff63bcea1ee)
Signed-off-by: Willy Tarreau <w@1wt.eu>
1 file changed