docs: add missing docstrings to output_engine/print_mode.py

[terriko]

We haven't been entirely consistent about adding python docstrings to every function, and I'd like to start improving the codebase. I'm filing separate tickets for each file I want updated so hacktoberfest folk can claim them more easily -- please only claim one or two at a time.

Here's a tutorial on docstrings in case you're not sure what they are. Basically I want someone to write a short sentence explaining what a function does and have it put at the top of the function in the way that python expects docstrings to look. (You can also do classes and methods.)

This issue is for updating the file foutput_engine/pdfbuilder.py. interrogate (a tool for checking docstrings) says it's missing the following docstrings:

interrogate -vv -i -I -M -C output_engine/print_mode.py

------------------------------ Detailed Coverage -------------------------------	Name	Status
print_mode.py
html_print_mode (L17)	MISSED
-----------------------------------------------	------------------------------

You will need to actually read the file and figure out what it does, then describe each function. Feel free to ask questions if you're not sure, or submit a partial fix where you only add docstrings to some functions.

Short tips for new contributors:

• cve-bin-tool's contributor docs
• If you've contributed to open source but not this project, you might just want our checklist for a great pull request
• cve-bin-tool uses https://www.conventionalcommits.org/ style for commit messages, and we have a test that checks the title of your pull request (PR). A good potential title for this one is in the title of this issue.
• You can make an issue auto close by including a comment "fixes #ISSUENUMBER" in your PR comments where ISSUENUMBER is the actual number of the issue. This "links" the issue to the pull request.

Claiming issues:

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[Ethaaan11]

could I be assigned to this please?

[aptitudepi]

@Ethaaan11 Are you still working on this?

[aptitudepi]

@terriko PR #3675 should fix this. Not entirely sure how to link it to this issue.

[aptitudepi]

@terriko Does PR #3677 work better? Also, may I please be assigned to this issue?