Clarify documentation of HFSS extent sizes

### Description of the modifications

The documentation for HFSS extent size properties (e.g. `airbox_horizontal`) is currently worded as follows:

"Airbox horizontal extent size. The first parameter is the value. The second parameter is a Boolean indicating if the value is a multiple."

This implies (to me, at least) that the second parameter should be `True` if the value is unitless (a multiple) and `False` if the value is an absolute length. However, the values in a test appear to be opposite this (here the second parameter is `False` but the value shown in AEDT is the multiple `0.15`):

### Useful links and references

_No response_

Search the documentation for HFSS extent size properties such as `airbox_horizontal`, then compare its wording with the test values described in the issue. Clarify whether the Boolean denotes a multiple or an absolute length, and ensure the documented explanation matches the AEDT behavior and example.

Written by the indexing model from the issue text.