doc: clarify documentation for rename/copy limits

A few places in the docs implied that rename/copy detection is always
quadratic or that all (unpaired) files were involved in the quadratic
portion of rename/copy detection.  The following two commits each
introduced an exception to this:

    9027f53 (Do linear-time/space rename logic for exact renames,
                  2007-10-25)
    bd24aa2 (diffcore-rename: guide inexact rename detection based
                  on basenames, 2021-02-14)

(As a side note, for copy detection, the basename guided inexact rename
detection is turned off and the exact renames will only result in
sources (without the dests) being removed from the set of files used in
quadratic detection.  So, for copy detection, the documentation was
closer to correct.)

Avoid implying that all files involved in rename/copy detection are
subject to the full quadratic algorithm.  While at it, also note the
default values for all these settings.

Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren

Documentation/config/diff.txt

@@ -118,9 +118,10 @@ diff.orderFile::
 	relative to the top of the working tree.
 
 diff.renameLimit::
-	The number of files to consider when performing the copy/rename
-	detection; equivalent to the 'git diff' option `-l`. This setting
-	has no effect if rename detection is turned off.
+	The number of files to consider in the exhaustive portion of
+	copy/rename detection; equivalent to the 'git diff' option
+	`-l`.  If not set, the default value is currently 400.  This
+	setting has no effect if rename detection is turned off.
 
 diff.renames::
 	Whether and how Git detects renames.  If set to "false",

Documentation/config/merge.txt

@@ -33,10 +33,12 @@ merge.verifySignatures::
 include::fmt-merge-msg.txt[]
 
 merge.renameLimit::
-	The number of files to consider when performing rename detection
-	during a merge; if not specified, defaults to the value of
-	diff.renameLimit. This setting has no effect if rename detection
-	is turned off.
+	The number of files to consider in the exhaustive portion of
+	rename detection during a merge.  If not specified, defaults
+	to the value of diff.renameLimit.  If neither
+	merge.renameLimit nor diff.renameLimit are specified,
+	currently defaults to 1000.  This setting has no effect if
+	rename detection is turned off.
 
 merge.renames::
 	Whether Git detects renames.  If set to "false", rename detection

Documentation/diff-options.txt

@@ -588,11 +588,16 @@ When used together with `-B`, omit also the preimage in the deletion part
 of a delete/create pair.
 
 -l<num>::
-	The `-M` and `-C` options require O(n^2) processing time where n
-	is the number of potential rename/copy targets.  This
-	option prevents rename/copy detection from running if
-	the number of rename/copy targets exceeds the specified
-	number.
+	The `-M` and `-C` options involve some preliminary steps that
+	can detect subsets of renames/copies cheaply, followed by an
+	exhaustive fallback portion that compares all remaining
+	unpaired destinations to all relevant sources.  (For renames,
+	only remaining unpaired sources are relevant; for copies, all
+	original sources are relevant.)  For N sources and
+	destinations, this exhaustive check is O(N^2).  This option
+	prevents the exhaustive portion of rename/copy detection from
+	running if the number of source/destination files involved
+	exceeds the specified number.  Defaults to diff.renameLimit.
 
 ifndef::git-format-patch[]
 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]::