[Docs] Add clarification to analysis example (#31826)

There have been at least two PRs trying to fix the spelling of "lazi" because it
isn't very clear from the example that the english analyzer will stem each token
in the example. This adds a short description of the analysis process to make
this clearer.

Relates to #31797

Author: Christoph Büscher

docs/reference/analysis.asciidoc

@@ -13,15 +13,18 @@ defined per index.
 [float]
 == Index time analysis
 
-For instance at index time, the built-in <<english-analyzer,`english`>> _analyzer_ would
-convert this sentence:
+For instance, at index time the built-in <<english-analyzer,`english`>> _analyzer_ 
+will first convert the sentence:
 
 [source,text]
 ------
 "The QUICK brown foxes jumped over the lazy dog!"
 ------
 
-into these terms, which would be added to the inverted index.
+into distinct tokens. It will then lowercase each token, remove frequent
+stopwords ("the") and reduce the terms to their word stems (foxes -> fox,
+jumped -> jump, lazy -> lazi). In the end, the following terms will be added
+to the inverted index:
 
 [source,text]
 ------