[gimp-help-2] Update mode and convert-grayscale documentation

[Elle Stone <ellestone src gnome org>]

commit 4b85e09374dc57a3e93668c010fac5813d974550
Author: Elle Stone <ellestone ninedegreesbelow com>
Date:   Sun Apr 2 09:29:40 2017 -0400

    Update mode and convert-grayscale documentation
    
    Updated screenshot added to image/mode.xml
    Updated image/convert-grayscale to include higher bit depths

 images/C/menus/image/mode.png         |  Bin 2305 -> 3050 bytes
 src/menus/image/convert-grayscale.xml |   34 +++++++++++++++++++++++++++++++-
 src/menus/image/mode.xml              |    1 +
 3 files changed, 33 insertions(+), 2 deletions(-)
---
diff --git a/images/C/menus/image/mode.png b/images/C/menus/image/mode.png
index 538b435..0d7768f 100644
Binary files a/images/C/menus/image/mode.png and b/images/C/menus/image/mode.png differ
diff --git a/src/menus/image/convert-grayscale.xml b/src/menus/image/convert-grayscale.xml
index d7686f9..ed1547e 100644
--- a/src/menus/image/convert-grayscale.xml
+++ b/src/menus/image/convert-grayscale.xml
@@ -2,6 +2,7 @@
 <!DOCTYPE sect2 PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
                        "http://www.docbook.org/xml/4.3/docbookx.dtd";>
 <!-- section history:
+  2017-04-01 elle stone: update contents for GIMP 2.10
   2007-03-08 alex falappa: added it translation
   2005/10/25 created from image-mode.xml by axel.wernicke
 -->
@@ -24,9 +25,38 @@
 
   <para>
     You can use the <guimenuitem>Grayscale</guimenuitem>
-    command to convert your image to grayscale with 256 levels of gray, from 0
-    (black) to 255 (white).
+    command to convert your image to grayscale. The resulting grayscale image
+    will have a single channel composed of various shades of gray ranging from
+    black to white.
   </para>
+  <para>
+    The number of availabe tonal steps between black and white depends on the
+    image precision:</para>
+      <orderedlist>
+      <listitem>
+      <para>At integer precision:</para>
+        <orderedlist>
+        <listitem><para>An 8-bit integer grayscale image provides 255
+        available tonal steps from 0 (black) to 255 (white).</para></listitem>
+        <listitem><para>A 16-bit integer grayscale image provides 65535
+        available tonal steps from 0 (black) to 65535 (white).</para></listitem>
+        <listitem><para>A 32-bit integer grayscale image theoretically will
+        provide 4294967295 tonal steps from 0 (black) to 4294967295 (white).
+        But as high bit depth GIMP 2.10 does all internal processing at 32-bit
+        floating point precision, the actual number of steps will be no more
+        than the number of tonal steps available in a 32-bit floating point
+        image.</para></listitem>
+        </orderedlist>
+    </listitem>
+    <listitem>
+    <para>At floating point precision the available number of tonal steps in a
+    grayscale image depends on the specified bit depth
+    (8-bit, 16-bit, or 32-bit) and the type of floating point that is
+    requested by the program (see
+    <ulink url="https://en.wikipedia.org/wiki/Floating_point";>
+      Floating-point arithmetic</ulink> for details).</para>
+    </listitem>
+      </orderedlist>
 
   <sect3>
     <title>Activating the Command</title>
diff --git a/src/menus/image/mode.xml b/src/menus/image/mode.xml
index c2481ae..a2d35e0 100644
--- a/src/menus/image/mode.xml
+++ b/src/menus/image/mode.xml
@@ -2,6 +2,7 @@
 <!DOCTYPE sect2 PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
                        "http://www.docbook.org/xml/4.3/docbookx.dtd";>
 <!-- section history:
+  2017-04-01 elle stone: add new image
   2007-04-01 KoSt: added 'no' translation
   2007-03-13 alex falappa: fixed links in it translation
   2005-10-29 de reviewed by axel.wernicke