docs installation.md: describe how to pass options to the installer on Linux (#6839)

(This is a suggestion that was easy for me to make a PR for; if other
approaches are considered better, feel free to consider this as a FR for
those instead)

I'd feel more comfortable using the installer with the instructions in
this commit, since I'm uncomfortable with random scripts trying to
modify my system config (PATH in this case).

Currently, the installer seems to be the best way to install `uv` that
allows updating it on a system without Homebrew or `pipx`. I hope
somebody will provide similar instructions for Windows.

I considered recommending saving the script to a file and then running
that, but I think it's better to have fewer options in the instructions.
Most people who'd want to save the file would figure it out.

As an aside, I would personally appreciate if `uv` could be installed
easily with `cargo install` or `cargo binstall`, but a friendly script
that acts predictably is probably more useful for more people.


## Test Plan

I tested the command on my machine, but I did not test compiling the
docs (yet). If the CI does not compile the docs, I could test this a bit
later, or perhaps this would be easier for somebody who already has a
dev environment set up.

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

docs/getting-started/installation.md

@@ -76,6 +76,13 @@ disable this behavior, use `INSTALLER_NO_MODIFY_PATH`. For example:
 $ curl -LsSf https://astral.sh/uv/install.sh | env INSTALLER_NO_MODIFY_PATH=1 sh
 ```
 
+Using environment variables is recommended because they are consistent across platforms. However,
+options can be passed directly to the install script. For example, to see the available options:
+
+```console
+$ curl -LsSf https://astral.sh/uv/install.sh | sh -s -- --help
+```
+
 ### PyPI
 
 For convenience, uv is published to [PyPI](https://pypi.org/project/uv/).