Fill in the missing docs for compiletime package members

Author: anatoliykmetyuk

library/src/scala/compiletime/package.scala

@@ -4,6 +4,16 @@ import scala.quoted._
 
 package object compiletime {
 
+  /** Use this method when you have a type, do not have a value for it but want to
+   *  pattern match on it. For example, given a type `Tup <: Tuple`, one can
+   *  pattern-match on it as follows:
+   *  ```
+   *  erasedValue[Tup] match {
+   *    case _: EmptyTuple => ...
+   *    case _: h *: t => ...
+   *  }
+   *  ```
+   */
   erased def erasedValue[T]: T = ???
 
   /** The error method is used to produce user-defined compile errors during inline expansion.
@@ -38,10 +48,20 @@ package object compiletime {
   transparent inline def (inline self: StringContext) code (inline args: Any*): String =
     ${ dotty.internal.CompileTimeMacros.codeExpr('self, 'args) }
 
+  /** Same as `constValue` but returns a `None` if a constant value
+   *  cannot be constructed from the provided type. Otherwise returns
+   *  that value wrapped in `Some`.
+   */
   inline def constValueOpt[T]: Option[T] = ???
 
+  /** Given a constant, singleton type `T`, convert it to a value
+   *  of the same singleton type. For example: `assert(constValue[1] == 1)`.
+   */
   inline def constValue[T]: T = ???
 
+  /** Given a tuple type `(X1, ..., Xn)`, returns a tuple value
+   *  `(constValue[X1], ..., constValue[Xn])`.
+   */
   inline def constValueTuple[T <: Tuple]: Tuple.Widen[T]=
     val res =
       inline erasedValue[T] match