pw_bloat: Rename pw_size_report -> pw_size_diff

The existing pw_size_report target actually runs a size diff on two
binaries. In preparation for landing single-binary size reporting
targets, the template is renamed pw_size_diff to more clearly signal
its purpose.

Change-Id: I90d78ca2edaa39c3883b2c74726ccf6161cfad0d
Requires: pigweed-internal:30301
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/104767
Reviewed-by: Brandon Vu <[email protected]>
Commit-Queue: Alexei Frolov <[email protected]>
diff --git a/pw_docgen/docs.rst b/pw_docgen/docs.rst
index c722d0a..5528ea1 100644
--- a/pw_docgen/docs.rst
+++ b/pw_docgen/docs.rst
@@ -69,7 +69,7 @@
 * ``inputs``: Additional resources required for the docs (images, data files,
   etc.)
 * ``group_deps``: Other ``pw_doc_group`` targets required by this one.
-* ``report_deps``: Report card generating targets (e.g. ``pw_size_report``) on
+* ``report_deps``: Report card generating targets (e.g. ``pw_size_diff``) on
   which the docs depend.
 
 **Example**