Clarify that recipe revisions use all of the exported files, not just conanfile.py

In the docs it says that

> When a recipe is exported, Conan calculates a unique ID (revision). For every change, a new recipe revision (RREV) will be calculated. By default it will use the checksum hash of the recipe manifest.

But it turn out that it uses all of the exported files to compute the recipe revision hash, not just `conanfile.py`. There is a log message to that effect but it would be good to be explicit in the docs.