Clarify saving LZW-compressed TIFFs in the documentation

[pkkm]

The documentation about saving TIFF images says that

Valid compression methods are: None, "tiff_ccitt", "group3", "group4", "tiff_jpeg", "tiff_adobe_deflate", "tiff_thunderscan", "tiff_deflate", "tiff_sgilog", "tiff_sgilog24", "tiff_raw_16"

However, I can create LZW-compressed TIFFs by passing a "tiff_lzw" value:

image.save("image.tif", compression="tiff_lzw")

Since LZW is a commonly used compression method, I think it would be good to clarify what's going on here:

• Does it accidentally happen to work but with problems I'm not seeing?
• Is the support dependent on the user's software configuration?
• Is the value "tiff_lzw" officially supported and the fact that it isn't in the documentation is an oversight?

[radarhere]

My answer is that it is a documentation oversight, so I've created #5336.

[pkkm]

Thanks!

[kmilos]

Also, I suggest taking out tiff_deflate from this doc list while you're at it, as it should not be used for writing, see this TIFF tech note:

A proprietary ZIP/Flate compression code (0x80b2) has been used by some software vendors. This code should be considered obsolete. The compression used by the obsolete code is identical to that defined above. We recommend that TIFF implementations recognize and read the obsolete code, but only write the official compression code (8).

I'd even go a step further and reassign the writing key as well:

COMPRESSION_INFO_REV = {v: k for k, v in COMPRESSION_INFO.items()}
# See deprecation note in https://www.adobe.io/content/dam/udp/en/open/standards/tiff/TIFFphotoshop.pdf
COMPRESSION_INFO_REV["tiff_deflate"] = 8

[radarhere]

I've removed tiff_deflate in #5336, and created #5343 to change it to tiff_adobe_deflate when writing.