etc, docs: regenerate system gimprc and its manpage

docs/gimprc.5.in

@@ -353,6 +353,13 @@ Sets whether GIMP should create previews of layers and channels. Previews in
 the layers and channels dialog are nice to have but they can slow things down
 when working with large images.  Possible values are yes and no.
 
+.TP
+(group-layer-previews yes)
+
+Sets whether GIMP should create previews of layer groups. Layer group previews
+are more expensive than ordinary layer previews.  Possible values are yes and
+no.
+
 .TP
 (layer-preview-size medium)
 
@@ -976,7 +983,7 @@ When enabled, the online user manual will be used by the help system.
 Otherwise the locally installed copy is used.  Possible values are yes and no.
 
 .TP
-(user-manual-online-uri "http://docs.gimp.org/2.10")
+(user-manual-online-uri "https://docs.gimp.org/2.10")
 
 The location of the online user manual. This is used if 'user-manual-online'
 is enabled.  This is a string value.

etc/gimprc.in

@@ -285,6 +285,12 @@
 # 
 # (layer-previews yes)
 
+# Sets whether GIMP should create previews of layer groups. Layer group
+# previews are more expensive than ordinary layer previews.  Possible values
+# are yes and no.
+# 
+# (group-layer-previews yes)
+
 # Sets the preview size used for layers and channel previews in newly created
 # dialogs.  Possible values are tiny, extra-small, small, medium, large,
 # extra-large, huge, enormous and gigantic.
@@ -776,7 +782,7 @@
 # The location of the online user manual. This is used if
 # 'user-manual-online' is enabled.  This is a string value.
 # 
-# (user-manual-online-uri "http://docs.gimp.org/2.10")
+# (user-manual-online-uri "https://docs.gimp.org/2.10")
 
 # The window type hint that is set on dock windows and the toolbox window.
 # This may affect the way your window manager decorates and handles these