Further overhaul of package line comments.

core/sync/chan/doc.odin

@@ -1,6 +1,5 @@
 /*
-This package provides both high-level and low-level channel types
-for thread-safe communication.
+High-level and low-level channel types for thread-safe communication.
 
 While channels are essentially thread-safe queues under the hood,
 their primary purpose is to facilitate safe communication between

core/sync/doc.odin

@@ -1,11 +1,7 @@
 /*
-Synchronization primitives
+Various synchronization primitives useful to mediate threads' access to shared memory.
 
-This package implements various synchronization primitives that can be used to
-synchronize threads' access to shared memory.
-
-To limit or control the threads' access to shared memory typically the
-following approaches are used:
+To limit or control the threads' access to shared memory typically the following approaches are used:
 
 - Locks
 - Lock-free