Rollup merge of #36396 - athulappadan:Default-docs, r=bluss

Documentation of what Default does for each type

Addresses #36265
I haven't changed the following types due to doubts:

1)src/libstd/ffi/c_str.rs
2)src/libcore/iter/sources.rs
3)src/libcore/hash/mod.rs
4)src/libcore/hash/mod.rs
5)src/librustc/middle/privacy.rs

r? @steveklabnik

src/librustc/ty/layout.rs

@@ -45,6 +45,7 @@ pub struct TargetDataLayout {
 }
 
 impl Default for TargetDataLayout {
+    /// Creates an instance of `TargetDataLayout`.
     fn default() -> TargetDataLayout {
         TargetDataLayout {
             endian: Endian::Big,