[DOCS] Typo in Search speed (#91934)

* [DOCS] Typo in Search speed

The PR https://github.com/elastic/elasticsearch/pull/89782 introduced some broken tags to leak in the text

* Fix tags

* Make all headings discrete

Co-authored-by: Abdon Pijpelink <abdon.pijpelink@elastic.co>

docs/reference/how-to/search-speed.asciidoc

@@ -10,7 +10,7 @@ goes to the filesystem cache so that Elasticsearch can keep hot regions of the
 index in physical memory.
 
 [discrete]
-tag::readahead[]
+// tag::readahead[]
 === Avoid page cache thrashing by using modest readahead values on Linux
 
 Search can cause a lot of randomized read I/O. When the underlying block
@@ -35,7 +35,7 @@ as a transient setting). We recommend a value of `128KiB` for readahead.
 WARNING: `blockdev` expects values in 512 byte sectors whereas `lsblk` reports
 values in `KiB`. As an example, to temporarily set readahead to `128KiB`
 for `/dev/nvme0n1`, specify `blockdev --setra 256 /dev/nvme0n1`.
-end::readahead[]
+// end::readahead[]
 
 [discrete]
 === Use faster hardware
@@ -358,7 +358,7 @@ PUT index
 }
 --------------------------------------------------
 
-tag::warm-fs-cache[]
+// tag::warm-fs-cache[]
 [discrete]
 === Warm up the filesystem cache
 
@@ -372,7 +372,7 @@ depending on the file extension using the
 WARNING: Loading data into the filesystem cache eagerly on too many indices or
 too many files will make search _slower_ if the filesystem cache is not large
 enough to hold all the data. Use with caution.
-end::warm-fs-cache[]
+// end::warm-fs-cache[]
 
 [discrete]
 === Use index sorting to speed up conjunctions
@@ -424,6 +424,7 @@ be able to cope with `max_failures` node failures at once at most, then the
 right number of replicas for you is
 `max(max_failures, ceil(num_nodes / num_primaries) - 1)`.
 
+[discrete]
 === Tune your queries with the Search Profiler
 
 The {ref}/search-profile.html[Profile API] provides detailed information about
@@ -438,6 +439,7 @@ Because the Profile API itself adds significant overhead to the query,
 this information is best used to understand the relative cost of the various
 query components. It does not provide a reliable measure of actual processing time.
 
+[discrete]
 [[faster-phrase-queries]]
 === Faster phrase queries with `index_phrases`
 
@@ -446,6 +448,7 @@ indexes 2-shingles and is automatically leveraged by query parsers to run phrase
 queries that don't have a slop. If your use-case involves running lots of phrase
 queries, this can speed up queries significantly.
 
+[discrete]
 [[faster-prefix-queries]]
 === Faster prefix queries with `index_prefixes`
 
@@ -454,6 +457,7 @@ indexes prefixes of all terms and is automatically leveraged by query parsers to
 run prefix queries. If your use-case involves running lots of prefix queries,
 this can speed up queries significantly.
 
+[discrete]
 [[faster-filtering-with-constant-keyword]]
 === Use `constant_keyword` to speed up filtering