Allow overriding of docstrings for schema documentation

[StevenMaude]

Sometimes tables are reused in a schema other than where they were added. This can lead to the quirk of documentation written for the intended schema having incorrect references when that table is reused elsewhere.

For example, as noted in #1821, the emis schema uses the ons_deaths table from core. And the raw.emis schema uses the ons_deaths table from raw.core.

In both of these docstrings, the raw.core.ons_deaths table is referred to. This is correct for the core schema. But not the emis schema, which has raw.emis.ons_deaths. The actual underlying tables are identical, which is fine, but this doesn't show the canonically correct import: we prefer to not mix imports from different schemas (core, tpp, emis).