Update man pages for recent changes

Update man pages for recent changes

Squashed commit of the following:

commit a7b2c919a76aab4b709dc4eb45f04234a8745ad1
Author: Fabio Scotoni <34964387+fscoto@users.noreply.github.com>
Date:   Thu Dec 5 08:25:33 2019 +0100

    Documentation: fix crypto_sha512 prototype

commit 7204c59458ce57f0967b42fe7998a411a097da55
Author: Fabio Scotoni <34964387+fscoto@users.noreply.github.com>
Date:   Thu Dec 5 08:23:10 2019 +0100

    Documentation: follow rename of HMAC functions

    crypto_hmac_* were renamed to crypto_hmac_sha512_* in 32525e92.

commit 2068f01e28cd2a5a4e2bc5ae4dd1a161e469f835
Author: Fabio Scotoni <34964387+fscoto@users.noreply.github.com>
Date:   Wed Dec 4 09:12:41 2019 +0100

    Adjust intro for new functions

commit 9cea43d2fa38b7249babe74f1c04066b4eaf90fa
Author: Fabio Scotoni <34964387+fscoto@users.noreply.github.com>
Date:   Wed Dec 4 09:00:34 2019 +0100

    Copy symbolic links as such

    cp -r copies symbolic links by dereferencing the link,
    essentially causing massive duplication of the man pages when installed.

    cp -PR copies the symlinks in place so that we have the contents on disk
    only once.

commit 19afe7d0f864acae143b0568956851dadddf2edc
Author: Fabio Scotoni <34964387+fscoto@users.noreply.github.com>
Date:   Wed Dec 4 08:47:43 2019 +0100

    Hook optional/ man pages to build

commit 668921527f186249dc8c54c87c0727b68ca30348
Author: Fabio Scotoni <34964387+fscoto@users.noreply.github.com>
Date:   Wed Dec 4 08:32:02 2019 +0100

    Remove mentions of context in Chacha20 documentation

    Contexts for Chacha20 as a whole were removed entirely.

commit e539f22ee036375a0ca7a3de4c84f30ba50d2738
Author: Fabio Scotoni <34964387+fscoto@users.noreply.github.com>
Date:   Wed Dec 4 08:30:26 2019 +0100

    Remove crypto_{mem,zero}cmp man pages

    These were deprecated in 2.0.0 already,
    which means they're due for removal now.

commit 8b04f0380dc0b7b8e81bcee86184c8b0100ca14a
Author: Fabio Scotoni <34964387+fscoto@users.noreply.github.com>
Date:   Tue Dec 3 11:41:24 2019 +0100

    Document custom hash EdDSA functions

commit 5663c8e1a5031eb7921e13cd9b717a924a6e458b
Author: Fabio Scotoni <34964387+fscoto@users.noreply.github.com>
Date:   Tue Dec 3 09:58:42 2019 +0100

    Document Ed25519 functions

commit 19478589d8afc1b1e782d33b9d6053ae89714f8b
Author: Fabio Scotoni <34964387+fscoto@users.noreply.github.com>
Date:   Tue Dec 3 09:31:14 2019 +0100

    Document HMAC functions

commit c75e9474a57d738141d64630ba6b7c28bf999f80
Author: Fabio Scotoni <34964387+fscoto@users.noreply.github.com>
Date:   Tue Dec 3 09:09:46 2019 +0100

    Document SHA-512 functions

commit 4fee40d5dfe7f65494a9ac43ac5ce114361063fa
Author: Fabio Scotoni <34964387+fscoto@users.noreply.github.com>
Date:   Mon Dec 2 11:13:29 2019 +0100

    Update Chacha20 documentation