[Doc] Documentation for FDB.h

[tbkr]

Added some documentation for the C++-API of the FDB class. There are still some open questions linked to some of the functions.

This is a first draft.

[FussyDuck]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 64.10%. Comparing base (72fd6af) to head (3e69ae6).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #49 +/- ## ======================================== Coverage 64.10% 64.10% ======================================== Files 238 238 Lines 13656 13656 Branches 1313 1313 ======================================== Hits 8754 8754 Misses 4902 4902 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[ChrisspyB]

For readability, I'd definitely like a dash (or colon, or some kind of separator) after a \param's keyword. e.g.:

Before: \param doit flag for committing to the wipe (default is dry-run) After: \param doit - flag for committing to the wipe (default is dry-run)

Edit: I notice you did have that, and removed it. I personally strongly prefer the presence of a separator

[mcakircali]

For readability, I'd definitely like a dash (or colon, or some kind of separator) after a \param's keyword. e.g.:

Before: \param doit flag for committing to the wipe (default is dry-run) After: \param doit - flag for committing to the wipe (default is dry-run)

Edit: I notice you did have that, and removed it. I personally strongly prefer the presence of a separator

Your suggestion means - should be part of the description, which becomes problematic.

e.g., using vscode, there is already - in the mouse-over previews.. and if a - is part of description, it looks like - - in previews.

I would strongly suggest against using - manually as part of a description.