Clarify pg_rewind behaviour in case of no divergence

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/16/app-pgrewind.html
Description:

After the sentence

"all other files, including new relation files, configuration files, and WAL
segments, are copied in full."

we should add

"However, no files are copied if there is no divergence, i.e. if the source
data directory is found to be compatible with the target data directory: in
that case pg_rewind exits successfully without performing the
synchronization."