Improve wxColour documentation

In the class description mention that in addition to RGB, an alpha value
is stored as well. Remove incorrect description of operator=(), clarify
meaning of "on" parameter in MakeMono(), and make few minor text edits.

Closes https://github.com/wxWidgets/wxWidgets/pull/1411

interface/wx/colour.h

@@ -27,9 +27,9 @@ const unsigned char wxALPHA_OPAQUE = 0xff;
     @class wxColour
 
     A colour is an object representing a combination of Red, Green, and Blue
-    (RGB) intensity values, and is used to determine drawing colours. See the
+    (RGB) intensity values and an Alpha value, and is used to determine
-    entry for wxColourDatabase for how a pointer to a predefined, named colour
+    drawing colours. See the entry for wxColourDatabase for how a pointer to a predefined,
-    may be returned instead of creating a new colour.
+    named colour may be returned instead of creating a new colour.
 
     Valid RGB values are in the range 0 to 255.
 
@@ -128,7 +128,7 @@ public:
 
         The arguments @a colRGB and @a colRGBA should be of the form 0x00BBGGRR
         and 0xAABBGGRR respectively where @c 0xRR, @c 0xGG, @c 0xBB and @c 0xAA
-        are the values of the red, blue, green and alpha components.
+        are the values of the red, green, blue and alpha components.
 
         Notice the right-to-left order of components!
 
@@ -161,8 +161,8 @@ public:
     /**
         Return the perceived brightness of the colour.
 
-        This value is computed using the simple @code 0.299*R + 0.587*G +
+        This value is computed using the simple @code 0.299*R + 0.587*G + 0.114*B @endcode
-        0.114*B @endcode formula with the coefficients taken from the RGB to
+        formula with the coefficients taken from the RGB to
         YIQ conversion formula and @c R, @c G and @c B being the values of the
         corresponding colour channels normalized to 0..1 range, so that the
         return value is 0 for black and 1 for white.
@@ -196,7 +196,7 @@ public:
     virtual unsigned char Red() const;
 
     /**
-        Returns @true if the color can be described using RGB values, ie is solid,
+        Returns @true if the color can be described using RGB values, i.e. is solid,
         @false if it is a pattern (currently only possible on macOS)
     */
     virtual bool IsSolid() const;
@@ -224,12 +224,12 @@ public:
 
     /**
         Tests the inequality of two colours by comparing individual red, green, blue
-        colours and alpha values.
+        intensities and alpha values.
     */
     bool operator !=(const wxColour& colour) const;
 
     /**
-        Assignment operator, using a colour name to be found in the colour database.
+        Assignment operator.
 
         @see wxColourDatabase
     */
@@ -237,12 +237,12 @@ public:
 
     /**
         Tests the equality of two colours by comparing individual red, green, blue
-        colours and alpha values.
+        intensities and alpha values.
     */
     bool operator ==(const wxColour& colour) const;
 
     /**
-        Assign 0 or 255 to rgb out parameters.
+        Assigns the same value to @a r, @a g, @a b: 0 if @a on is @c false, 255 otherwise.
         @since 2.9.0
     */
     static void MakeMono(unsigned char* r, unsigned char* g, unsigned char* b, bool on);
@@ -284,10 +284,9 @@ public:
     static unsigned char AlphaBlend(unsigned char fg, unsigned char bg, double alpha);
 
     /**
-        ChangeLightness() is a utility function that simply darkens
+        Utility function that simply darkens or lightens a color, based on the specified 
-        or lightens a color, based on the specified percentage
+        percentage @a ialpha. @a ialpha of 0 would be make the color completely black, 
-        ialpha of 0 would be completely black, 200 completely white
+        200 completely white and 100 would not change the color.
-        an ialpha of 100 returns the same colour
         @since 2.9.0
     */
     static void ChangeLightness(unsigned char* r, unsigned char* g, unsigned char* b, int ialpha);