[ES|QL] Functions definitions updates (elastic#224993)

## Summary

I had to run it manually due to this ES change
elastic/elasticsearch#129839

Author: stratoula

src/platform/packages/private/kbn-language-documentation/src/sections/generated/aggregation_functions.tsx

@@ -305,7 +305,7 @@ export const functions = {
       label: i18n.translate('languageDocumentation.documentationESQL.st_centroid_agg', {
         defaultMessage: 'ST_CENTROID_AGG',
       }),
-      preview: false,
+      preview: true,
       description: (
         <Markdown
           openLinksInNewTab
@@ -336,7 +336,7 @@ export const functions = {
       label: i18n.translate('languageDocumentation.documentationESQL.st_extent_agg', {
         defaultMessage: 'ST_EXTENT_AGG',
       }),
-      preview: false,
+      preview: true,
       description: (
         <Markdown
           openLinksInNewTab

src/platform/packages/private/kbn-language-documentation/src/sections/generated/operators.tsx

@@ -237,13 +237,9 @@ export const functions = {
             {
               defaultMessage: `
   ### IS NOT NULL
-  Use \`IS NOT NULL\` to filter data based on whether the field exists or not.
+  Returns \`false\` if the value is \`NULL\`, \`true\` otherwise.
 
-  \`\`\`esql
-  FROM employees
-  | WHERE is_rehired IS NOT NULL
-  | STATS COUNT(emp_no)
-  \`\`\`
+  Note: If a field is only in some documents it will be \`NULL\` in the documents that did not contain it.
   `,
               description:
                 'Text is in markdown. Do not translate function names, special characters, or field names like sum(bytes)',
@@ -269,12 +265,9 @@ export const functions = {
             {
               defaultMessage: `
   ### IS NULL
-  Use \`IS NULL\` to filter data based on whether the field exists or not.
+  Returns \`true\` if the value is \`NULL\`, \`false\` otherwise.
 
-  \`\`\`esql
-  FROM employees
-  | WHERE birth_date IS NULL
-  \`\`\`
+  Note: If a field is only in some documents it will be \`NULL\` in the documents that did not contain it.
   `,
               description:
                 'Text is in markdown. Do not translate function names, special characters, or field names like sum(bytes)',
@@ -386,7 +379,7 @@ export const functions = {
       label: i18n.translate('languageDocumentation.documentationESQL.match_operator (:)', {
         defaultMessage: 'MATCH_OPERATOR (:)',
       }),
-      preview: true,
+      preview: false,
       description: (
         <Markdown
           openLinksInNewTab