doc: erasure code ghobject is made of gen_t + shard_t

Signed-off-by: Loic Dachary <loic@dachary.org>
mon: fix typo in comment

Signed-off-by: Loic Dachary <loic@dachary.org>
mon: fix typo and remove redundant sentence

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #555 from dachary/master

minor documentation fixes