Better specify discoveryDependencies and monitorDependencies fields in package_index.json

Author: cmaglie

docs/package_index_json-specification.md

@@ -247,13 +247,15 @@ Each PLATFORM describes a core for a specific architecture. The fields needed ar
   platform. Each tool is referenced by the triple (`packager`, `name`, `version`) as previously said. Note that you can
   reference tools available in other packages as well, even if no platform of that package is installed.
 - `discoveryDependencies`: the Pluggable Discoveries needed by this platform. Each discovery is referenced by the pair
-  (`packager`, `name`), the `version` is not specified because the latest installed discovery tool will always be used.
-  Like `toolsDependencies` they will be installed by Boards Manager along with the platform and can reference tools
-  available in other packages as well, even if no platform of that package is installed.
+  (`packager`, `name`). The Pluggable Discoveries must be part of `tools` section of the platform. The `version` is not
+  specified because the latest available discovery tool will always be used. Like `toolsDependencies` they will be
+  installed by Boards Manager along with the platform and can reference tools available in other packages as well, even
+  if no platform of that package is installed.
 - `monitorDependencies`: the Pluggable Monitors needed by this platform. Each monitor is referenced by the pair
-  (`packager`, `name`), the `version` is not specified because the latest installed monitor tool will always be used.
-  Like `toolsDependencies` they will be installed by Boards Manager along with the platform and can reference tools
-  available in other packages as well, even if no platform of that package is installed.
+  (`packager`, `name`). The Pluggable Monitors must be part of `tools` section of the platform. The `version` is not
+  specified because the latest available monitor tool will always be used. Like `toolsDependencies` they will be
+  installed by Boards Manager along with the platform and can reference tools available in other packages as well, even
+  if no platform of that package is installed.
 
 The `version` field is validated by both Arduino IDE and [JSemVer](https://github.com/zafarkhaja/jsemver). Here are the
 rules Arduino IDE follows for parsing versions