fix: update documentation to reflect preference for --source over --rev

I should have updated the documentation in 5205b46bd887 (fix: add a -s
option to format a revision and its descendants, 2020-03-13) and/or
a6ef1e8e2f6d (fix: mark -r as advanced, 2020-03-13)...

Differential Revision: https://phab.mercurial-scm.org/D8808

hgext/fix.py

@@ -241,15 +241,15 def fix(ui, repo, *pats, **opts):
     of files, unless the --whole flag is used. Some tools may always affect the
     whole file regardless of --whole.
 
-    If revisions are specified with --rev, those revisions will be checked, and
-    they may be replaced with new revisions that have fixed file content.  It is
-    desirable to specify all descendants of each specified revision, so that the
-    fixes propagate to the descendants. If all descendants are fixed at the same
-    time, no merging, rebasing, or evolution will be required.
+    If --working-dir is used, files with uncommitted changes in the working copy
+    will be fixed. Note that no backup are made.
 
-    If --working-dir is used, files with uncommitted changes in the working copy
-    will be fixed. If the checked-out revision is also fixed, the working
-    directory will update to the replacement revision.
+    If revisions are specified with --source, those revisions and their
+    descendants will be checked, and they may be replaced with new revisions
+    that have fixed file content. By automatically including the descendants,
+    no merging, rebasing, or evolution will be required. If an ancestor of the
+    working copy is included, then the working copy itself will also be fixed,
+    and the working copy will be updated to the fixed parent.
 
     When determining what lines of each file to fix at each revision, the whole
     set of revisions being fixed is considered, so that fixes to earlier

tests/test-fix.t

@@ -84,15 +84,15 Help text for fix.
       lines of files, unless the --whole flag is used. Some tools may always
       affect the whole file regardless of --whole.
   
-      If revisions are specified with --rev, those revisions will be checked,
-      and they may be replaced with new revisions that have fixed file content.
-      It is desirable to specify all descendants of each specified revision, so
-      that the fixes propagate to the descendants. If all descendants are fixed
-      at the same time, no merging, rebasing, or evolution will be required.
+      If --working-dir is used, files with uncommitted changes in the working
+      copy will be fixed. Note that no backup are made.
   
-      If --working-dir is used, files with uncommitted changes in the working
-      copy will be fixed. If the checked-out revision is also fixed, the working
-      directory will update to the replacement revision.
+      If revisions are specified with --source, those revisions and their
+      descendants will be checked, and they may be replaced with new revisions
+      that have fixed file content. By automatically including the descendants,
+      no merging, rebasing, or evolution will be required. If an ancestor of the
+      working copy is included, then the working copy itself will also be fixed,
+      and the working copy will be updated to the fixed parent.
   
       When determining what lines of each file to fix at each revision, the
       whole set of revisions being fixed is considered, so that fixes to earlier