doc: v8 functions as methods on v8

For consistency with other docs, show v8 functions as being on the v8
object.

PR-URL: #6615
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>

doc/api/v8.md

@@ -6,7 +6,7 @@ This module exposes events and interfaces specific to the version of [V8][]
 built with Node.js.  These interfaces are subject to change by upstream and are
 therefore not covered under the stability index.
 
-## getHeapStatistics()
+## v8.getHeapStatistics()
 
 Returns an object with the following properties
 
@@ -21,7 +21,7 @@ Returns an object with the following properties
 }
 ```
 
-## getHeapSpaceStatistics()
+## v8.getHeapSpaceStatistics()
 
 Returns statistics about the V8 heap spaces, i.e. the segments which make up
 the V8 heap. Order of heap spaces nor availability of a heap space can be
@@ -70,7 +70,7 @@ Example result:
 ]
 ```
 
-## setFlagsFromString(string)
+## v8.setFlagsFromString(string)
 
 Set additional V8 command line flags.  Use with care; changing settings
 after the VM has started may result in unpredictable behavior, including