Explain why certain tables can be skipped in search-replace (#221)

Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: Copilot

README.md

@@ -23,6 +23,10 @@ multisite, this will just be the tables for the current site unless
 Search/replace intelligently handles PHP serialized data, and does not
 change primary key values.
 
+Tables without a primary key are skipped. To check whether a table has a
+primary key, run `wp db query 'DESCRIBE <table>'` and look for 'PRI' in
+the Key column.
+
 **OPTIONS**
 
 	[<old>]

src/Search_Replace_Command.php

@@ -140,6 +140,10 @@ class Search_Replace_Command extends WP_CLI_Command {
 	 * Search/replace intelligently handles PHP serialized data, and does not
 	 * change primary key values.
 	 *
+	 * Tables without a primary key are skipped. To check whether a table has a
+	 * primary key, run `wp db query 'DESCRIBE <table>'` and look for 'PRI' in
+	 * the Key column.
+	 *
 	 * ## OPTIONS
 	 *
 	 * [<old>]