doc: tweak Captures documentation

This was suggested [on Discord](https://discord.com/channels/273534239310479360/1120175689124036669/1181401471720370237).

Author: BurntSushi

src/regex/bytes.rs

@@ -1568,10 +1568,15 @@ impl<'h> From<Match<'h>> for core::ops::Range<usize> {
 
 /// Represents the capture groups for a single match.
 ///
-/// Capture groups refer to parts of a regex enclosed in parentheses. They can
-/// be optionally named. The purpose of capture groups is to be able to
-/// reference different parts of a match based on the original pattern. For
-/// example, say you want to match the individual letters in a 5-letter word:
+/// Capture groups refer to parts of a regex enclosed in parentheses. They
+/// can be optionally named. The purpose of capture groups is to be able to
+/// reference different parts of a match based on the original pattern. In
+/// essence, a `Captures` is a container of [`Match`] values for each group
+/// that participated in a regex match. Each `Match` can be looked up by either
+/// its capture group index or name (if it has one).
+///
+/// For example, say you want to match the individual letters in a 5-letter
+/// word:
 ///
 /// ```text
 /// (?<first>\w)(\w)(?:\w)\w(?<last>\w)

src/regex/string.rs

@@ -1573,10 +1573,15 @@ impl<'h> From<Match<'h>> for core::ops::Range<usize> {
 
 /// Represents the capture groups for a single match.
 ///
-/// Capture groups refer to parts of a regex enclosed in parentheses. They can
-/// be optionally named. The purpose of capture groups is to be able to
-/// reference different parts of a match based on the original pattern. For
-/// example, say you want to match the individual letters in a 5-letter word:
+/// Capture groups refer to parts of a regex enclosed in parentheses. They
+/// can be optionally named. The purpose of capture groups is to be able to
+/// reference different parts of a match based on the original pattern. In
+/// essence, a `Captures` is a container of [`Match`] values for each group
+/// that participated in a regex match. Each `Match` can be looked up by either
+/// its capture group index or name (if it has one).
+///
+/// For example, say you want to match the individual letters in a 5-letter
+/// word:
 ///
 /// ```text
 /// (?<first>\w)(\w)(?:\w)\w(?<last>\w)