[PATCH v3 5/5] gitdiffcore doc: mention new preliminary step for rename detection

@@ -168,6 +168,23 @@ a similarity score different from the default of 50% by giving a
 number after the "-M" or "-C" option (e.g. "-M8" to tell it to use
 8/10 = 80%).
 
+Note that when rename detection is on but both copy and break
+detection are off, rename detection adds a preliminary step that first
+checks if files are moved across directories while keeping their
+filename the same.  If there is a file added to a directory whose
+contents is sufficiently similar to a file with the same name that got
+deleted from a different directory, it will mark them as renames and
+exclude them from the later quadratic step (the one that pairwise
+compares all unmatched files to find the "best" matches, determined by
+the highest content similarity).  So, for example, if
+docs/extensions.txt and docs/config/extensions.txt have similar
+content, then they will be marked as a rename even if it turns out
+that docs/extensions.txt was more similar to src/extension-checks.c.
+At most, one comparison is done per file in this preliminary pass; so
+if there are several extensions.txt files throughout the directory
+hierarchy that were added and deleted, this preliminary step will be
+skipped for those files.
+
 Note.  When the "-C" option is used with `--find-copies-harder`
 option, 'git diff-{asterisk}' commands feed unmodified filepairs to
 diffcore mechanism as well as modified ones.  This lets the copy