meson: Fix doc installation path computation

This applies the logic from 8f6858064b (append "postgresql" to some
installation paths if it does not already contain "pgsql" or
"postgres") also to the doc installation directory.

Reviewed-by: Tristan Partin <tristan@neon.tech>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/b018b577-38a2-49c6-8727-adfb577de317@eisentraut.org

meson.build

@@ -511,7 +511,10 @@ endif
 dir_man = get_option('mandir')
 
 # FIXME: These used to be separately configurable - worth adding?
-dir_doc = get_option('datadir') / 'doc' / 'postgresql'
+dir_doc = get_option('datadir') / 'doc'
+if not (dir_prefix_contains_pg or dir_doc.contains('pgsql') or dir_doc.contains('postgres'))
+  dir_doc = dir_doc / pkg
+endif
 dir_doc_html = dir_doc / 'html'
 
 dir_locale = get_option('localedir')