go/parser: Better comment for ParseFile.
go/parser: Better comment for ParseFile.

The name ParseFile was kept as it indicates that
this function parses a Go 'SourceFile' per the
Go spec. Similarly, the other functions (ParseExpr,
ParseStmtList, ParseDeclList) parse the corresponding
constructs as defined in the Go spec.

Fixes issue 1311.

R=r, rsc
CC=golang-dev
http://codereview.appspot.com/3453042

glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,scirelli/scirelli-go,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,scirelli/scirelli-go,mhennings/marcohennings-go,bryanxu/go-zh,npe9/noahevans-go9,webfd/go-zh,scirelli/scirelli-go,d0f/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,scirelli/scirelli-go,mhennings/marcohennings-go,bryanxu/go-zh,scirelli/scirelli-go,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,d0f/go-zh,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,webfd/go-zh,npe9/noahevans-go9,d0f/go-zh,npe9/noahevans-go9,npe9/noahevans-go9,bryanxu/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,webfd/go-zh,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,npe9/noahevans-go9,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,d0f/go-zh,d0f/go-zh,Triskite/willstone-goclone,mhennings/marcohennings-go,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,Triskite/willstone-goclone,bryanxu/go-zh,Triskite/willstone-goclone,npe9/noahevans-go9,npe9/noahevans-go9,bryanxu/go-zh,npe9/noahevans-go9,mhennings/marcohennings-go,bryanxu/go-zh,scirelli/scirelli-go,Triskite/willstone-goclone,bryanxu/go-zh,scirelli/scirelli-go,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,rflanagan/reginaldflanagan-project1