Fix doc warnings

[mamcx]

Description of Changes

Fix as many warnings caused by cargo doc.

Some that stay:

• Of the form warning: public documentation for X links to private item Y
• The top doc on crates/core/src/subscription/subscription.rs This generate unresolved link warnings but looks to me that need a full rewrite c.c. @joshua-spacetime.

Expected complexity level and risk

0

Testing

• [x] Run cargo doc --all. Because we have a naming collision:

Error: document output filename collision
The lib `spacetimedb` in package `spacetimedb-core v1.0.0-rc4 (.../space/SpacetimeDB/crates/core)` has the same name as the lib `spacetimedb` in package `spacetimedb v1.0.0-rc4 (../space/SpacetimeDB/crates/bindings)`.
Only one may be documented at once since they output to the same path.

... then I switch adding doc =false in each to let me see all the warnings.

[jdetter]

I don't have any input on this one, feel free to continue without me 👍

[CLAassistant]

All committers have signed the CLA.