doc: update rz-asm manual with formatting improvements and environment variables

[akshvy]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository.
• I made sure to follow the project's coding style.
• I've documented every RZ_API function and struct this PR changes.
• I've added tests that prove my changes are effective (required for changes to RZ_API).
• I've updated the Rizin book with the relevant information (if needed).
• I've used AI tools to generate fully or partially these code changes and I'm sure the changes are not copyrighted by somebody else.

Detailed description

This PR improves the rz-asm.1 manpage by fixing formatting inconsistencies and cleaning up the environment variables section.

Changes:

• Fixed Environment Section: Removed literal \n" and stray characters from RZ_DEBUG and RZ_NOPLUGINS descriptions.
• Improved Macros: Converted the plain text environment list to use proper .Bl -tag and .It Ev macros for correct terminal rendering.
• Header Standardization: Updated section headers (like DIRECTIVES) to standard uppercase.
• Date Update: Set the document date to the current revision date.

AI Disclosure: Used Gemini to assist with correct mdoc/roff macro syntax for the environment section.

Test plan

I verified the changes locally by rendering the manpage in the terminal:

man -l binrz/man/rz-asm.1

[Rot127]

Please add a linter job in the CI. Otherwise it is hard to verify that the changes work.

[Rot127]

This is not how you would test it.
In general we do not accept vibe coded contributions.
Which your PR and commits here very much look like.

Please see our CONTRIBUTING.md for requirements for new contributors.
The testing requirement is in your case, that you need to add a CI job to test the mandoc warnings.