Custom Fields: update "is searchable" behaviour is now "optimize for search"

Related core changes:

• https://github.com/civicrm/civicrm-core/pull/30186 : Display all custom fields in Reports
• https://github.com/civicrm/civicrm-core/pull/30209 : Display all custom fields in the Advanced Search
• https://github.com/civicrm/civicrm-core/pull/30188 : Change label/help for "Is Searchable?"

More discussion, including advice on database indexes:
dev/core#1735 (closed)

This section of the docs should be updated to reflect the above:

• Update the screenshot here, which mentions "Is Searchable?" https://docs.civicrm.org/user/en/latest/organising-your-data/creating-custom-fields/#custom-fields
• Update the paragraph in the section about "Is Searchable" https://docs.civicrm.org/user/en/latest/organising-your-data/creating-custom-fields/#is-this-field-searchable