Separate sections for range/slice pattern capturing

The rules for the capturing behavior related to range patterns and
slice patterns were comingled with the rules for capturing and
discriminant reads.  While these are spiritually related, they are
distinct, so let's break these apart into separate sections.

Author: traviscross

src/types/closure.md

@@ -353,7 +353,11 @@ let _ = &mut x; // ERROR: Cannot borrow `x` as mutable.
 c();
 ```
 
-r[type.closure.capture.precision.discriminants.range-patterns]
+
+r[type.closure.capture.precision.range-patterns]
+### Capturing and range patterns
+
+r[type.closure.capture.precision.range-patterns.reads]
 Matching against a [range pattern][patterns.range] reads the place being matched, even if the range includes all possible values of the type, and captures the place by `ImmBorrow`.
 
 ```rust,compile_fail,E0502
@@ -365,7 +369,10 @@ let _ = &mut x; // ERROR: Cannot borrow `x` as mutable.
 c();
 ```
 
-r[type.closure.capture.precision.discriminants.slice-patterns-slices]
+r[type.closure.capture.precision.slice-patterns]
+### Capturing and slice patterns
+
+r[type.closure.capture.precision.slice-patterns.slices]
 Matching a slice against a [slice pattern][patterns.slice] other than one with only a single [rest pattern][patterns.rest] (i.e. `[..]`) is treated as a read of the length from the slice and captures the slice by `ImmBorrow`.
 
 ```rust,compile_fail,E0502
@@ -392,7 +399,7 @@ let _ = &mut *x; // OK: `*x` can be borrow here.
 c();
 ```
 
-r[type.closure.capture.precision.discriminants.slice-patterns-arrays]
+r[type.closure.capture.precision.slice-patterns.arrays]
 As the length of an array is fixed by its type, matching an array against a slice pattern does not by itself capture the place.
 
 ```rust,no_run