Document --max large-window decompression

[Rohithmatham12]

Fixes #4687.
Fixes #4688.

This updates the CLI help and documentation around maximum compression settings:

• list --max in the advanced CLI help and README help block
• update the README thread default wording so it no longer implies -T1
• document that frames created with a window log larger than 27 can also be decompressed by passing --max

Test plan:

make -j4 zstd
programs/zstd -H
git diff --check