[skip changelog] Add documentation for library components of the gRPC interface (#692)

* [skip changelog] Add documentation for library components of the gRPC interface

Comments added to the .proto files are included in the generated gRPC interface documentation.

* Correct documentation re: relative vs absolute paths provided by Library message fields

Co-authored-by: Roberto Sora <r.sora@arduino.cc>

* Use preferred "platform" term in gRPC interface documentation

The official term is now "platform", rather than the previous ambiguous "core".

Co-authored-by: Roberto Sora <r.sora@arduino.cc>

* Fix formatting of gRPC interface documentation

* Fix incorrect documentation for task_progress field of LibraryUninstall response

* Document that library version is the key of the releases map

* Update gRPC interface code comments

Co-authored-by: Roberto Sora <r.sora@arduino.cc>

Author: per1234

rpc/commands/commands.pb.go

@@ -96,18 +96,27 @@ service ArduinoCore {
   // List all installed platforms.
   rpc PlatformList(PlatformListReq) returns (PlatformListResp);
 
+  // Download the archive file of an Arduino library in the libraries index to
+  // the staging directory.
   rpc LibraryDownload(LibraryDownloadReq) returns (stream LibraryDownloadResp);
 
+  // Download and install an Arduino library from the libraries index.
   rpc LibraryInstall(LibraryInstallReq) returns (stream LibraryInstallResp);
 
+  // Uninstall an Arduino library.
   rpc LibraryUninstall(LibraryUninstallReq) returns (stream LibraryUninstallResp);
 
+  // Upgrade all installed Arduino libraries to the newest version available.
   rpc LibraryUpgradeAll(LibraryUpgradeAllReq) returns (stream LibraryUpgradeAllResp);
 
+  // List the recursive dependencies of a library, as defined by the `depends`
+  // field of the library.properties files.
   rpc LibraryResolveDependencies(LibraryResolveDependenciesReq) returns (LibraryResolveDependenciesResp);
 
+  // Search the Arduino libraries index for libraries.
   rpc LibrarySearch(LibrarySearchReq) returns (LibrarySearchResp);
 
+  // List the installed libraries.
   rpc LibraryList(LibraryListReq) returns (LibraryListResp);
 }