docs(graphcache): Update cache update docs

[kitten]

Summary

We have to clarify in our docs why cache updates may be written and when they can be avoided. This can trivially be done by explaining what cache updaters are for.

Additionally, while we now support partial, optimistic updaters, we don't actually clarify that this is possible in the docs, and instead state that it still isn't. The docs now state that we don't have to define all fields.

Set of changes

• Add section on cache updates
• Add section on partial, optimistic updates