Update documentation regarding species sources and normalization

[KiranEiden]

PR summary

This PR updates the Hydrodynamics section documentation to highlight that sources for passive variables in the conserved state are off by default. It also removes the reference to castro.normalize_species, which does not appear to be a valid runtime parameter any longer. I am happy to introduce additional documentation regarding the species normalization if that would be helpful -- my understanding is that it is on by default now.

PR checklist

• [ ] test suite needs to be run on this PR
• [ ] this PR will change answers in the test suite to more than roundoff level
• [ ] all newly-added functions have docstrings as per the coding conventions
• [ ] the CHANGES file has been updated, if appropriate
• [ ] if appropriate, this change is described in the docs

[zingale]

Oh, I missed that there was only a single `

can you fix the typesetting of USE_SPECIES_SOURCES in a new PR?

[KiranEiden]

Sure, sorry about that. I’ll do that later tonight.