Clarify main discriminant reads rule

The rule,

> If pattern matching requires inspecting a discriminant, the relevant
> place will get captured by `ImmBorrow`.

is unclear in two ways.  For one, when does pattern matching "require
inspecting" the discriminant?  Second, what is the "relevant" place?

Let's break this up into two rules (and add rule identifiers).  The
first rule will state that reading the discriminant captures the place
containing the discriminant (by `ImmBorrow`).  The second rule will
state explicitly the baseline rule for when such reads happen.

Author: traviscross

src/types/closure.md

@@ -288,7 +288,11 @@ let c = || {
 r[type.closure.capture.precision.discriminants]
 ### Capturing for discriminant reads
 
-If pattern matching requires inspecting a discriminant, the relevant place will get captured by `ImmBorrow`.
+r[type.closure.capture.precision.discriminants.reads]
+If pattern matching reads a discriminant, the place containing that discriminant is captured by `ImmBorrow`.
+
+r[type.closure.capture.precision.discriminants.multiple-variant]
+Matching against a variant of an enum that has more than one variant reads the discriminant, capturing the place by `ImmBorrow`.
 
 ```rust
 enum Example {