Improve indexing on the chapter/section/subsection levels.

 doc/lispref/windows.texi (Recombining Windows): Index subject of sections.
 doc/lispref/variables.texi (Variables with Restricted Values)
 (Generalized Variables): Index subject of sections.
 doc/lispref/text.texi (Buffer Contents, Examining Properties)
 (Changing Properties, Property Search, Substitution): Index
 subject of sections.
 doc/lispref/syntax.texi (Motion and Syntax, Parsing Expressions)
 (Motion via Parsing, Position Parse, Control Parsing): Index
 subject of sections.
 doc/lispref/strings.texi (Predicates for Strings, Creating Strings)
 (Modifying Strings, Text Comparison): Index subject of sections.
 doc/lispref/searching.texi (Syntax of Regexps, Regexp Special)
 (Regexp Functions, Regexp Functions): Index subject of sections.
 doc/lispref/processes.texi (Subprocess Creation, Process Information): Index
 subject of sections.
 doc/lispref/positions.texi (Screen Lines): Index subject of sections.
 doc/lispref/nonascii.texi (Scanning Charsets, Specifying Coding Systems):
 Index subject of sections.
 doc/lispref/minibuf.texi (Text from Minibuffer, Object from Minibuffer)
 (Multiple Queries, Minibuffer Contents): Index subject of
 sections.
 doc/lispref/markers.texi (Predicates on Markers, Creating Markers)
 (Information from Markers, Moving Markers): Index subject of
 sections.
 doc/lispref/macros.texi (Defining Macros, Problems with Macros): Index
 subject of sections.
 doc/lispref/loading.texi (Loading Non-ASCII, Where Defined): Index subject
 of sections.
 doc/lispref/lists.texi (List-related Predicates, List Variables, Setcar)
 (Setcdr, Plist Access): Index subject of sections.
 doc/lispref/keymaps.texi (Controlling Active Maps, Scanning Keymaps)
 (Modifying Menus): Index subject of sections.
 doc/lispref/help.texi (Accessing Documentation, Help Functions): Index
 subject of sections.
 doc/lispref/hash.texi (Hash Access): Index subject of sections.
 doc/lispref/functions.texi (Core Advising Primitives)
 (Advising Named Functions, Porting old advices): Index subject of
 sections.
 doc/lispref/frames.texi (Creating Frames, Initial Parameters)
 (Position Parameters, Buffer Parameters, Minibuffers and Frames)
 (Pop-Up Menus, Drag and Drop): Index subject of sections.
 doc/lispref/files.texi (Visiting Functions, Kinds of Files)
 (Unique File Names): Index subject of sections.
 doc/lispref/display.texi (Refresh Screen, Echo Area Customization)
 (Warning Variables, Warning Options, Delayed Warnings)
 (Temporary Displays, Managing Overlays, Overlay Properties)
 (Finding Overlays, Size of Displayed Text, Defining Faces)
 (Attribute Functions, Displaying Faces, Face Remapping)
 (Basic Faces, Font Lookup, Fontsets, Replacing Specs)
 (Defining Images, Showing Images): Index subject of sections.
 doc/lispref/debugging.texi (Debugging, Explicit Debug)
 (Invoking the Debugger, Excess Open, Excess Close): Index subject
 of sections.
 doc/lispref/customize.texi (Defining New Types, Applying Customizations)
 (Custom Themes): Index subject of sections.
 doc/lispref/control.texi (Sequencing, Combining Conditions)
 (Processing of Errors, Cleanups): Index subject of sections.
 doc/lispref/compile.texi (Eval During Compile): Index subject of sections.
 doc/lispref/commands.texi (Using Interactive, Distinguish Interactive)
 (Command Loop Info, Classifying Events, Event Mod)
 (Invoking the Input Method): Index subject of sections.
 doc/lispref/buffers.texi (Buffer List, Buffer Gap): Index subject of sections.
 doc/lispref/backups.texi (Making Backups, Numbered Backups, Backup Names)
 (Reverting): Index subject of sections.
 doc/lispref/abbrevs.texi (Abbrev Tables, Defining Abbrevs, Abbrev Files)
 (Abbrev Expansion, Standard Abbrev Tables, Abbrev Properties)
 (Abbrev Table Properties): Index subject of sections.
 doc/lispref/os.texi (Time of Day, Time Conversion, Time Parsing)
 (Time Calculations, Idle Timers): Index subject of sections.