Update Channels XML comment

[kmgallahan]

Indicate that only color channels are returned.

Description

I was scratching my head as to why ChannelCount and Channels.Count() weren't matching until I viewed the source.

[dlemstra]

Thanks again for helping out with improving the documentation. The property also returns meta channels so I wonder if we could use another word than color.

[kmgallahan]

Ah yes, I missed that. So colors plus:

MetaPixelChannels = 10,

and anything else added later below 64, but excluding:

IndexPixelChannel = 5,
ReadMaskPixelChannel = 6,
WriteMaskPixelChannel = 7,
MetaPixelChannel = 8, /* deprecated */
CompositeMaskPixelChannel = 9,
IntensityPixelChannel = MaxPixelChannels,  /* ???? */
CompositePixelChannel = MaxPixelChannels,  /* ???? */
SyncPixelChannel = MaxPixelChannels+1      /* not a real channel */

Not sure how to phrase that. Do you remember what drove the decision to filter these channels? That could inform the description.

[dlemstra]

I did not include the index and mask channels because they are used for a specific functionality in the library and don't really change how the image looks? And the other ones are either deprecated or only used on the command line. I really appreciate you making this more clear for future users.

[kmgallahan]

Ok, I just added metadata. I was considering adding Excludes index and mask channels, but I don't think listing exclusions is particularly common in API docs.