Alter order of precedence and upated docs to match

docs/running_psalm/configuration.md

@@ -416,6 +416,13 @@ Whether or not to allow `require`/`include` calls in your PHP. Defaults to `true
 ```
 Allows you to hard-code a serializer for Psalm to use when caching data. By default, Psalm uses `ext-igbinary` *if* the version is greater than or equal to 2.0.5, otherwise it defaults to PHP's built-in serializer.
 
+#### threads
+```xml
+<psalm
+        threads="[int]"
+>
+```
+Allows you to hard-code the number of threads Psalm will use (similar to `--threads` on the command line). This value will be used in place of detecting threads from the host machine, but will be overridden by using `--threads` or `--debug` (which sets threads to 1) on the command line
 
 ## Project settings
 

src/Psalm/Internal/Cli/Psalm.php

@@ -246,7 +246,7 @@ final class Psalm
             $options['long-progress'] = true;
         }
 
-        $threads = self::detectThreads($options, $in_ci);
+        $threads = self::useThreads($options, $config, $in_ci);
 
         self::emitMacPcreWarning($options, $threads);
 
@@ -909,12 +909,14 @@ final class Psalm
         }
     }
 
-    private static function detectThreads(array $options, bool $in_ci): int
+    private static function detectThreads(array $options, Config $config, bool $in_ci): int
     {
         if (isset($options['threads'])) {
             $threads = (int)$options['threads'];
         } elseif (isset($options['debug']) || $in_ci) {
             $threads = 1;
+        } elseif ($config->threads) {
+            $threads = $config->threads;
         } else {
             $threads = max(1, ProjectAnalyzer::getCpuCount() - 1);
         }
@@ -1170,4 +1172,15 @@ final class Psalm
             );
         }
     }
+
+    private static function useThreads(array $options, bool $in_ci, Config $config): int
+    {
+        $threads = self::detectThreads($options, $in_ci);
+
+        if ($config->threads && $config->threads<$threads) {
+            $threads = $config->threads;
+        }
+
+        return $threads;
+    }
 }