Document specifying the PCM solvent by name

[JustinKyleKirkland]

Documentation for pyscf/pyscf#3416, as requested by @jeanwsr in that PR.

That change lets the PCM family take a solvent name, which assigns both the static dielectric constant eps and the optical one eps_optical. The user guide previously described only assigning eps by hand, and never mentioned eps_optical at all, even though it is what the non-equilibrium solvation of excited states actually uses.

Changes to source/user/solvent.rst

The PCM Solvent parameters section is rewritten around the name-based API and now covers:

• the three places a name can be given: the PCM constructor, the .PCM() method of a mean-field object, and assignment to with_solvent.solvent at any point;
• case-insensitive matching that ignores spaces and hyphens, and the recognized abbreviations (h2o, dmso, dmf, dma, thf, dcm, ccl4, chcl3, mecn, acn, meoh, etoh, et2o, ether);
• what the two dielectric constants each control, and that eps_optical matters only for excited states;
• that without a name, eps_optical falls back to water and a warning is issued when eps indicates a different solvent.

The manual eps route is kept, since it still works and still overrides the database value.

The Solvent for excited states section gains a short pointer noting that the slow solvent uses eps_optical, so specifying the solvent by name is the right way to set up those calculations.

The attribution of the parameters is also brought in line with the merged database: they come from the Minnesota Solvent Descriptor Database, except for the dielectric constant of water, which is the 78.3553 value from gaussian.com/scrf.

Checks

The two new internal cross references resolve against unique section titles, and docutils reports no warnings for the file beyond the pre-existing note about the _user_solvent label being referenced from other pages.

This should be merged after pyscf/pyscf#3416, since it documents behaviour that only exists there.