Add special type format strings to docs

[God-damnit-all]

Adds the notes from 0038a8c to the documentation. I changed 'extended' to 'special' for clarity.

Also, in the format specifier table, I changed 'before` and 'after' to 'start' and 'end' to keep this from happening:

Here's a direct link to the modified document:
https://github.com/ImportTaste/gallery-dl/blob/docs/formatting.md/docs/formatting.md#special-type-format-strings

[mikf]

Thank you. Hope you don't mind if I also tinker with this for a bit myself, add more examples, etc. {_env[…]} and {_now} should also be included in this file, I think.

Also, in the format specifier table, I changed 'before` and 'after' to 'start' and 'end' to keep this from happening:

Looked like this for me:

[God-damnit-all]

Thank you. Hope you don't mind if I also tinker with this for a bit myself, add more examples, etc. {_env[…]} and {_now} should also be included in this file, I think.

Also, in the format specifier table, I changed 'before` and 'after' to 'start' and 'end' to keep this from happening:

Looked like this for me:

Of course I don't mind, though I'm not sure if the env variables should be considered a 'special type' format string since \f isn't involved. Considering you just added 46e17c5, maybe they could be considered 'local variable fields'?

EDIT: Actually, I think system environment fields works even better. Or maybe even local system fields, though that is a bit redundant. local client fields? Though system environment fields is still my best suggestion, I feel.