ESQL: Add counters to signature for IS NULL (elastic#129670)

This adds `counter_long` and `counter_double` to the signatures of
supported fields for `IS NULL` and `IS NOT NULL`. We hadn't been
generating those signatures since the docs v3 migration, so this had to
plug those in. In addition, it changes the wording on a few things and
adds a note that if a field is only in some documents then the ones
missing the field will have `NULL` - which is important information for
`IS NULL` and `IS NOT NULL`.

Author: nik9000

docs/reference/query-languages/esql/_snippets/operators/detailedDescription/is_not_null.md

@@ -0,0 +1,6 @@
+
+::::{note}
+If a field is only in some documents it will be `NULL` in the documents that did not contain it.
+::::
+
+

docs/reference/query-languages/esql/_snippets/operators/detailedDescription/is_null.md

@@ -0,0 +1,6 @@
+
+::::{note}
+If a field is only in some documents it will be `NULL` in the documents that did not contain it.
+::::
+
+

docs/reference/query-languages/esql/_snippets/operators/types/is_not_null.md

@@ -7,6 +7,9 @@
 | boolean | boolean |
 | cartesian_point | boolean |
 | cartesian_shape | boolean |
+| counter_double | boolean |
+| counter_integer | boolean |
+| counter_long | boolean |
 | date | boolean |
 | date_nanos | boolean |
 | double | boolean |

docs/reference/query-languages/esql/_snippets/operators/types/is_null.md

@@ -7,6 +7,9 @@
 | boolean | boolean |
 | cartesian_point | boolean |
 | cartesian_shape | boolean |
+| counter_double | boolean |
+| counter_integer | boolean |
+| counter_long | boolean |
 | date | boolean |
 | date_nanos | boolean |
 | double | boolean |