added docstrings for quoteof() and tile()

Author: xitology

doc/src/api.md

@@ -3,4 +3,6 @@
 ```@docs
 PrettyPrinting.pprint
 PrettyPrinting.pprintln
+PrettyPrinting.quoteof
+PrettyPrinting.tile
 ```

doc/src/guide.md

@@ -281,8 +281,8 @@ implementing function `tile()`, which should map an object to the corresponding
 layout expression.
 
 Continuing with the type `Node` defined in section [Extending
-PrettyPrinting](#Extending-PrettyPrinting), let us give it a custom layout
-generated with `list_layout()`.
+PrettyPrinting](@ref), let us give it a custom layout generated with
+`list_layout()`.
 
     import PrettyPrinting: tile
 

src/PrettyPrinting.jl

@@ -53,4 +53,24 @@ function pprintln(io::IO, data)
     println(io)
 end
 
+"""
+    quoteof(obj)
+
+Convert an object to its representation in Julia AST.
+
+Implement this method to customize [`pprint`](@ref) on a user-defined type.
+"""
+function quoteof
+end
+
+"""
+    tile(obj)
+
+Convert an object to the corresponding layout expression.
+
+Implement this method to customize [`pprint`](@ref) on a user-defined type.
+"""
+function tile
+end
+
 end