Simplify install notes

[kleisauke]

Noticed this after I wrote comment #507 (comment). However, this is not an issue for editable installs. For example, this is all fine:

$ pip uninstall -y pyvips pyvips-binary
$ pip cache purge
$ pip install -e .[binary]
$ ldd $(python -m site --user-site)/_libvips.abi3.so | grep libvips
	libvips-65e2e1b0.so.42 => /home/kleisauke/.local/lib/python3.13/site-packages/pyvips_binary.libs/libvips-65e2e1b0.so.42 (0x00007f1d86c00000)

$ pip uninstall -y pyvips pyvips-binary
$ pip cache purge
$ pip install pyvips-binary
$ pip install -e .
$ ldd $(python -m site --user-site)/_libvips.abi3.so | grep libvips
	libvips-65e2e1b0.so.42 => /home/kleisauke/.local/lib/python3.13/site-packages/pyvips_binary.libs/libvips-65e2e1b0.so.42 (0x00007f1d86c00000)

But this builds against a globally installed libvips, if present.

$ pip uninstall -y pyvips pyvips-binary
$ pip cache purge
$ pip install .[binary]
$ ldd $(python -m site --user-site)/_libvips.abi3.so | grep libvips
	libvips.so.42 => /lib64/libvips.so.42 (0x00007fa0f3c00000)

$ pip uninstall -y pyvips pyvips-binary
$ pip cache purge
$ pip install pyvips-binary
$ pip install .
$ ldd $(python -m site --user-site)/_libvips.abi3.so | grep libvips
	libvips.so.42 => /lib64/libvips.so.42 (0x00007f5d8a200000)

[jcupitt]

Ooop, sorry, I missed this PR.