doc: improve child_process doc types

PR-URL: https://github.com/nodejs/node/pull/8741
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/child_process.md

@@ -1046,7 +1046,7 @@ this occurs.
 added: v0.1.90
 -->
 
-* {Stream}
+* {Stream.Readable}
 
 A `Readable Stream` that represents the child process's `stderr`.
 
@@ -1061,7 +1061,7 @@ the same value.
 added: v0.1.90
 -->
 
-* {Stream}
+* {Stream.Writable}
 
 A `Writable Stream` that represents the child process's `stdin`.
 
@@ -1119,7 +1119,7 @@ assert.equal(child.stdio[2], child.stderr);
 added: v0.1.90
 -->
 
-* {Stream}
+* {Stream.Readable}
 
 A `Readable Stream` that represents the child process's `stdout`.