Added documentation for MS-DOS and Win32 timestamp conversions

[TheN00bBuilder]

Your checklist for this pull request

• [v] I've read the guidelines for contributing to this repository
• [v] I made sure to follow the project's coding style
• [v] I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• [v] I've added tests that prove my fix is effective or that my feature works (if possible)
• [v] I've updated the rizin book with the relevant information (if needed)

Detailed description

I'm really sorry folks, I did not see the manpages under binrz/man until wargio pointed it out in my book PR (rizinorg/book#123). This PR adds the documentation for the commands added in #4526.

I would have written / included it in the last PR but did not know of this location for the manpages. Going forward, I'll be sure to check this directory and others for their manpages.

Test plan

None needed.

Closing issues

None.