Adjust docs of few toxencrypt function to the code

[Diadlo]

Add correct information about behaviour if input pointer is NULL

Fix #590

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This change is 

[zoff99]

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Reviewed 1 of 1 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable

[nurupo]

You shouldn't edit toxencryptsave.h directly, you should edit toxencryptsave.api.h and run APIDSL on it and then run astyle on the resulting toxencryptsave.h using toxcore's astyle rules.

[Diadlo]

@nurupo IDK what APIDSL is and how does it works. But I hope, that now it's made correct

[nurupo]

APIDSL converts files written in a special c-like language that @iphydf came up with, the goal of the language being improving API consistency, to the C language. E.g. (tox.api.h)[https://github.com/TokTok/c-toxcore/blob/master/toxcore/tox.api.h] gets converted into (tox.h)[https://github.com/TokTok/c-toxcore/blob/master/toxcore/tox.h]. I think the plan is to eventually port all public APIs and also toxcore layers to it. There is also this doc on it, though I remember APIDSL on Heroku being outdated, so it might be better to build the latest version of it locally.

Here are the astyle rules.

There is a tests Travis runs that checks if someone forgot to run astyle or edited a header file without editing the corresponding APIDSL file accordingly, which is why your previous change was failing on Travis.

[nurupo]

Hm, actually, the tests still fails, toxencryptsave.h and toxencryptsave.api.h are still out of sync in this PR.

[Diadlo]

Thanks for the links. Fixed

[zoff99]

@nurupo maybe you can add somewhere in the docs that if you compile libsodium with
"--enable-minimal"
then 99,9% of tox works (including all A/V stuff) but only 1 function is missing for toxencryptsave
that one will only be built with the full version of libsodium
(function crypto_pwhash_scryptsalsa208sha256)

[robinlinden]

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Reviewed 1 of 1 files at r1, 1 of 1 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable

[robinlinden]

Please enable the checkbox "Allow edits from maintainers." on the bottom right.

[Diadlo]

@robinlinden Done