Further overhaul of package line comments.

core/c/c.odin

@@ -1,4 +1,4 @@
-// package c just defines the basic types used by C programs.
+// Defines the basic types used by `C` programs for foreign function and data structure interop.
 package c
 
 import builtin "base:builtin"

core/c/libc/doc.odin

@@ -1,2 +1,2 @@
-// package libc declares the commonly used things in "libc" (C standard library).
+// Declares the commonly used things in `libc` (`C` standard library).
 package libc