[docs] Naming on GDB/MI sections in docs

[Vladimir Prus <vladimir@codesourcery.com>]

At present, the toc of documentation PDF looks like this:

	gdb
	27 The gdb/mi Interface
	Function and Purpose
	Notation and Terminology
	27.3 gdb/mi General Design
	27.3.1 Context management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
	27.3.2 Asynchronous command execution and non-stop mode . .
	27.3.3 Thread groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
	27.4 gdb/mi Command Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
	27.4.1 gdb/mi Input Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I have two concerns about it:

1. Why some section titles are started by "gdb/mi"? We're already inside "The gdb/mi Interface"
section, so it's reasonable to assume subsections talk about gdb/mi. Can we remove those
prefixes?

2. Why is "Function and Purpose" subsection lacks section number in the TOC? I would
have expected "27.1".

Thanks,
Volodya