Update documentation to reflect new expression phase terminology

[geoffromer]

#2964 changed our expression phase terminology:

• "value phase" -> "expression phase"
• "symbolic value" -> "symbolic constant"
• "constant" -> "template constant"
• "constant or symbolic value" -> "constant"

However, some documentation still uses the term "symbolic value", and some uses of the term "constant" appear to be following the old definition (e.g. here).

Cleaning up "symbolic value" should be a simple find-and-replace. Cleaning up "constant" probably has to be done by manual inspection, since some uses are presumably using the new definition, but the blame history may help: any usage older than July 2023 (when #2964 was merged) very likely needs to be rewritten to "template constant".

[geoffromer]

But note that we've been shifting toward "concrete constant" instead of "template constant" in the toolchain, although that hasn't yet been codified in a proposal.