Document approach in CMEW developer documentation for recipes that want a subset of available period

Prerequisite: #456

Some recipes in CMEW should only use a particular period or number of years, due to e.g. like-for-like comparison with observations. There is more than one way to constrain this in an ESMValTool recipe, via e.g. the dataset specification or by diagnostic/variable. We need to agree with owning scientists how to constrain this for particular recipes in a manner that plays nice with CMEW. E.g. we may decide that CMEW will specify the dataset with no time constraint in all recipes and therefore the recipe needs to constrain elsewhere.

Whatever is decided should be documented in the CMEW developer documentation.

Review prerequisite #456 and the CMEW developer documentation. First agree with the owning scientists whether period limits belong in the ESMValTool dataset specification or at diagnostic/variable level; document the agreed CMEW-compatible approach for recipes and define completion as that guidance being added.

Written by the indexing model from the issue text.