Add note about used context in the model selector (#15235)

When pressing <kbd>control</kbd> + <kbd>enter</kbd>, the AI-powered
inline transformation input displays an icon button and a token count,
which should show roughly the same numbers you'd see on your assistant
panel. At a first glance, though, the token count not being zero can be
confusing, where you'd wonder where that's coming from. That's because
the inline input uses whatever piece of context and/or information of
the currently selected assistant tab to suggest more accurate edits.

So, this PR introduces an informative piece of text to the
`ModelSelector` menu, on the inline transformation input, which delivers
exactly this bit of info, aimed at clarifying the connection between
these two methods of interacting with LLMs.

I've also took the opportunity to change the icon button's icon to one
that's a bit easier to see, still representing the affordance of "click
to configure something".

Release Notes:

- Add note about how inline edits consume context from the assistant
panel to clarify interaction with LLMs.

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

assets/icons/sliders-alt.svg

@@ -0,0 +1,6 @@
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+<circle cx="4" cy="10" r="1.875" stroke="black" stroke-width="1.75"/>
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