Clarify use of `execute` versus `load` in documentation
Clarify use of `execute` versus `load` in documentation

Adds cross-references to the documentation of the `execute` and `load`
functions that help clarify when it's appropriate to use each one, and
suggests the use of the other when it's not.

Fixes #1136.

diesel-rs/diesel,diesel-rs/diesel,sgrif/diesel,sgrif/diesel