doc .gif->.png for Scribble Latex output

svn: r13009

original commit: 7b3c5db0f8a9f5915ca88b69b2178be8c349e57f

collects/scribblings/scribble/manual.scrbl

@@ -943,7 +943,11 @@ combination of @scheme[envvar] and @scheme[as-index].}
 
  The path is relative to the current directory, which is set by
  @exec{setup-plt} and @exec{scribble} to the directory of the main
- document file.}
+ document file.
+
+ When generating Latex output, if the filename has a @filepath{.gif}
+ suffix, then the suffix is changed to @filepath{.png} (so a PNG file
+ must exist in addition to the GIF file).}
 
 @defproc[(image/plain [filename-relative-to-source string?]
                       [pre-element any/c] ...)

collects/scribblings/scribble/struct.scrbl

@@ -666,7 +666,10 @@ layer is a style for the hyperlink.}
 
 Used as a style for an @scheme[element] to inline an image. The
 @scheme[path] field can be a result of
-@scheme[path->main-collects-relative].}
+@scheme[path->main-collects-relative].
+
+For Latex output, a @filepath{.gif} suffix on @scheme[path] is
+replaced with a @filepath{.png} suffix.}
 
 
 @defproc[(block? [v any/c]) boolean?]{