add some missing bitmap% and dc<%> docs

collects/scribblings/draw/bitmap-class.scrbl

@@ -269,6 +269,19 @@ A monochrome bitmap saved as @scheme['png] without a mask bitmap
 
 }
 
+@defmethod[(set-argb-pixels [x real?]
+                            [y real?]
+                            [width exact-nonnegative-integer?]
+                            [height exact-nonnegative-integer?]
+                            [pixels bytes?]
+                            [alpha? any/c #f])
+           void?]{
+
+The same as @xmethod[bitmap-dc% set-argb-pixels], but the
+bitmap does not have to be selected into the DC.
+
+}
+
 @defmethod[(set-gl-config [config (is-a?/c gl-config%)])
            void?]{
 

collects/scribblings/draw/bitmap-dc-class.scrbl

@@ -47,8 +47,8 @@ The same as @method[dc<%> draw-bitmap-section]. In older version, this
 
 @defmethod[(get-argb-pixels [x real?]
                             [y real?]
-                            [width (integer-in 1 10000)]
+                            [width exact-nonnegative-integer?]
-                            [height (integer-in 1 10000)]
+                            [height exact-nonnegative-integer?]
                             [pixels (and/c bytes? (not/c immutable?))]
                             [alpha? any/c #f])
            void?]{
@@ -102,8 +102,8 @@ result is @scheme[#f].
 
 @defmethod[(set-argb-pixels [x real?]
                             [y real?]
-                            [width (integer-in 1 10000)]
+                            [width exact-nonnegative-integer?]
-                            [height (integer-in 1 10000)]
+                            [height exact-nonnegative-integer?]
                             [pixels bytes?]
                             [alpha? any/c #f])
            void?]{

collects/scribblings/draw/dc-intf.scrbl

@@ -26,7 +26,7 @@ available for caching text-extent information.}
            void?]{
 
 Clears the drawing region (fills it with the current background color,
-as determined by @method[dc<%> get-background]).
+as determined by @method[dc<%> get-background]). See also @method[dc<%> erase].
 
 }
 
@@ -447,6 +447,15 @@ For printer or PostScript output, an exception is raised if
 }
 
 
+@defmethod[(erase)
+           void?]{
+
+Erases the drawing region by filling it with white and, for a drawing
+context that keeps an alpha channels, sets all alphas to zero.
+
+}
+
+
 @defmethod[(flush) void?]{
 
 Calls the @xmethod[canvas<%> flush] method for