Add missing docs page

Author: MilesCranmer

docs/src/symbolic_units.md

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+# Symbolic Dimensions
+
+Whereas `u"..."` will automatically convert all units to the same
+base SI units, `us"..."` will not. This uses the `SymbolicDimensions`
+type, which is a subtype of `AbstractDimensions` that stores the
+dimensions symbolically. This is useful for keeping track of the
+original units and constants in a user-entered expression.
+
+The two main functions for working with symbolic
+units are `sym_uparse` and `us_str`:
+
+```@docs
+@us_str
+sym_uparse
+```
+
+To convert a quantity to its regular base SI units, use `expand_units`:
+
+```@docs
+expand_units
+```