doc: correct checkHost behavior with wildcards etc

The current documentation is inaccurate in that checkHost does not
necessarily return the given host name, but instead returns the subject
name that matched the given host name.

Refs: nodejs#36804

PR-URL: nodejs#41468
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Filip Skokan <panva.ip@gmail.com>

doc/api/crypto.md

@@ -2498,11 +2498,17 @@ added: v15.6.0
   * `partialWildcards` {boolean} **Default:** `true`.
   * `multiLabelWildcards` {boolean} **Default:** `false`.
   * `singleLabelSubdomains` {boolean} **Default:** `false`.
-* Returns: {string|undefined} Returns `name` if the certificate matches,
-  `undefined` if it does not.
+* Returns: {string|undefined} Returns a subject name that matches `name`,
+  or `undefined` if no subject name matches `name`.
 
 Checks whether the certificate matches the given host name.
 
+If the certificate matches the given host name, the matching subject name is
+returned. The returned name might be an exact match (e.g., `foo.example.com`)
+or it might contain wildcards (e.g., `*.example.com`). Because host name
+comparisons are case-insensitive, the returned subject name might also differ
+from the given `name` in capitalization.
+
 ### `x509.checkIP(ip[, options])`
 
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