doc: use 'object database' not ODB or abbreviation

doc: use 'object database' not ODB or abbreviation

The abbreviation 'ODB' is used in the technical documentation
sections for commit-graph and parallel-checkout, along with an
'odb' option in `git-pack-redundant`, without expansion.

Use 'object database' in full, in those entries. The text has not
been reflowed to keep the changes minimal.

While in the glossary for `object` terms, add the common`oid`
abbreviation to its entry.

Signed-off-by: Philip Oakley <redacted>
Signed-off-by: Taylor Blau <redacted>