[8.x] ESQL: Improve LOOKUP JOIN page (#125688)

Some minor edits, formatting improvements etc.

docs/reference/esql/esql-lookup-join.asciidoc

@@ -61,7 +61,7 @@ contains multi-valued entries, those entries will not match anything
 [[esql-lookup-join-example]]
 ==== Example
 
-`LOOKUP JOIN` has left-join behavior. If no rows match in the looked index, `LOOKUP JOIN` retains the incoming row and adds `null`s. If many rows in the lookedup index match, `LOOKUP JOIN` adds one row per match.
+`LOOKUP JOIN` has left-join behavior. If no rows match in the lookup index, `LOOKUP JOIN` retains the incoming row and adds nulls. If many rows in the lookup index match, `LOOKUP JOIN` adds one row per match.
 
 In this example, we have two sample tables:
 
@@ -151,8 +151,11 @@ as the same type in {esql}
 `int`)
 *** `float`, `half_float`, and `scaled_float` are compatible
 with `double` (all represented as `double`)
-** For text fields: You can use text fields on the left-hand side of the
-join only if they have a `.keyword` subfield
+** For text fields: You can only use text fields as the join key on the
+left-hand side of the join and only if they have a `.keyword` subfield
+
+To obtain a join key with a compatible type, use a
+<<esql-type-conversion-functions,conversion function>> if needed.
 
 For a complete list of supported data types and their internal
 representations, see the <<esql-supported-types,Supported Field Types documentation>>.
@@ -164,14 +167,14 @@ representations, see the <<esql-supported-types,Supported Field Types documentat
 The following are the current limitations with `LOOKUP JOIN`
 
 * Indices in <<index-mode-setting,lookup>> mode are always single-sharded.
-* Cross cluster search is unsupported. Both source and lookup indices
+* Cross cluster search is unsupported initially. Both source and lookup indices
 must be local.
 * Currently, only matching on equality is supported.
 * `LOOKUP JOIN` can only use a single match field and a single index.
 Wildcards, aliases, datemath, and datastreams are not supported.
-* The name of the match field in `LOOKUP JOIN lu_idx ON match_field` must match
-an existing field in the query. This may require `RENAME`s or `EVAL`s to
-achieve.
+* The name of the match field in
+`LOOKUP JOIN lu++_++idx ON match++_++field` must match an existing field
+in the query. This may require renames or evals to achieve.
 * The query will circuit break if there are too many matching documents
 in the lookup index, or if the documents are too large. More precisely,
 `LOOKUP JOIN` works in batches of, normally, about 10,000 rows; a large