Update units.rst

[seanpearsonuk]

See https://github.com/ansys/pyfluent/blame/main/doc/source/user_guide/units.rst. The current text begins:

Physical units

By default, you can access and modify PyFluent solver settings according to its S.I. unit.
However, PyFluent also allows you to work in arbitrary physical examples. Here's an example:

---

Actions

• Fix typo: arbitrary physical examples -> arbitrary physical units.
• Add any caveats. E.g., the TUI API works in terms of the current units in Fluent as specified by the user.
• Write SI instead of S.I. (old-fashioned).

[ansjschuetz]

May I please add the following request?

In https://github.com/ansys/pyfluent/blob/main/doc/source/user_guide/legacy/tui.rst commands of the form "solver_session.tui...." are discussed.

Here it is important to draw the reader's attention to the fact that, while all (most?) other PyFluent commands always expect SI units, these commands expect "user units", i.e. if the user e.g. sets length units to "mm", all (or at least most) "solver_session.tui...." commands will also expect lengths to be specified in "mm", not "m".