docs, etc: regenerate the default gimprc and its manpage

docs/gimprc.5.in

@@ -320,7 +320,7 @@ kept available until the undo-size limit is reached.  This is an integer
 value.
 
 .TP
-(undo-size 1565502976)
+(undo-size 1g)
 
 Sets an upper limit to the memory that is used per image to keep operations on
 the undo stack. Regardless of this setting, at least as many undo-levels as

etc/gimprc.in

@@ -262,7 +262,7 @@
 # bytes, kilobytes, megabytes or gigabytes. If no suffix is specified the
 # size defaults to being specified in kilobytes.
 # 
-# (undo-size 1565502976)
+# (undo-size 1g)
 
 # Sets the size of the previews in the Undo History.  Possible values are
 # tiny, extra-small, small, medium, large, extra-large, huge, enormous and