doc: match link text in index to doc headline

The index ToC says "About these docs" but the document itself says
"About this documentation" which I think is better. Use that.

PR-URL: #34449
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

doc/api/index.md

@@ -5,7 +5,7 @@
 
 <!--introduced_in=v0.10.0-->
 
-* [About these docs](documentation.html)
+* [About this documentation](documentation.html)
 * [Usage and example](synopsis.html)
 
 <div class="line"></div>