docs: update unit tests watch description

Descriptions for the "watch" option where not clear.

(cherry picked from commit 8df6fcd)

Author: alan-agius4

packages/angular/build/src/builders/karma/schema.json

@@ -229,7 +229,7 @@
     },
     "watch": {
       "type": "boolean",
-      "description": "Run build when files change.",
+      "description": "Re-run tests when source files change.",
       "default": true
     },
     "poll": {

packages/angular/build/src/builders/unit-test/schema.json

@@ -44,7 +44,7 @@
     },
     "watch": {
       "type": "boolean",
-      "description": "Run build when files change."
+      "description": "Re-run tests when source files change. Defaults to `true` in TTY environments and `false` otherwise."
     },
     "debug": {
       "type": "boolean",