ID 252531 - Add information about Op::input_format() not being thread safe to the NDK

Problem summary
Add information about Op::input_format() not being thread safe to the NDK

Reproduced by support

https://www.thefoundry.co.uk/products/nuke/developers/100/ndkdevguide/advanced/opscommon.html?highlight=input_format

Expected behaviour
For there to be more information about Op::input_format()

Actual behaviour
There isn't