Document otlLib.builder, high-level lookup builders.

Essentially the documentation from #1995, incorporates @dscorbett
feedback.

Doc/source/otlLib/builder.rst

@@ -1,8 +0,0 @@
-#######
-builder
-#######
-
-.. automodule:: fontTools.otlLib.builder
-   :inherited-members:
-   :members:
-   :undoc-members:

Doc/source/otlLib/index.rst

@@ -1,13 +1,74 @@
-######
-otlLib
-######
+#################################################
+otlLib: Routines for working with OpenType Layout
+#################################################
 
-.. toctree::
-   :maxdepth: 2
+The ``fontTools.otlLib`` library provides routines to help you create the
+subtables and other data structures you need when you are editing a font's
+``GSUB`` and ``GPOS`` tables: substitution and positioning rules, anchors,
+lookups, coverage tables and so on.
 
-   builder
+------------------------------------------
+High-level OpenType Layout Lookup Builders
+------------------------------------------
 
-.. automodule:: fontTools.otlLib
-   :inherited-members:
-   :members:
-   :undoc-members:
+.. automodule:: fontTools.otlLib.builder
+   :members: AlternateSubstBuilder, ChainContextPosBuilder, ChainContextSubstBuilder, LigatureSubstBuilder, MultipleSubstBuilder, CursivePosBuilder, MarkBasePosBuilder, MarkLigPosBuilder, MarkMarkPosBuilder, ReverseChainSingleSubstBuilder, SingleSubstBuilder, ClassPairPosSubtableBuilder, PairPosBuilder, SinglePosBuilder
+   :member-order: bysource
+
+--------------------------------------
+Common OpenType Layout Data Structures
+--------------------------------------
+
+.. automodule:: fontTools.otlLib.builder
+   :members: buildCoverage, buildLookup
+
+------------------------------------
+Low-level GSUB Table Lookup Builders
+------------------------------------
+
+These functions deal with the "simple" lookup types. See above for classes to
+help build more complex lookups (contextual and chaining lookups).
+
+.. automodule:: fontTools.otlLib.builder
+   :members: buildSingleSubstSubtable, buildMultipleSubstSubtable, buildAlternateSubstSubtable, buildLigatureSubstSubtable
+
+--------------------------
+GPOS Shared Table Builders
+--------------------------
+
+The functions help build the `GPOS shared tables <https://docs.microsoft.com/en-us/typography/opentype/spec/gpos#shared-tables-value-record-anchor-table-and-mark-array-table>`_
+as defined in the OpenType spec: value records, anchors, mark arrays and
+mark record tables.
+
+.. automodule:: fontTools.otlLib.builder
+   :members: buildValue, buildAnchor, buildMarkArray, buildDevice, buildBaseArray, buildComponentRecord, buildMarkArray, buildValue
+   :member-order: bysource
+
+------------------------------------
+Low-level GPOS Table Lookup Builders
+------------------------------------
+
+These functions deal with the "simple" lookup types. See above for classes to
+help build more complex lookups (contextual and chaining lookups).
+
+.. automodule:: fontTools.otlLib.builder
+   :members: buildCursivePosSubtable, buildLigatureArray, buildMarkBasePos, buildMarkBasePosSubtable, buildMarkLigPos, buildMarkLigPosSubtable, buildPairPosClassesSubtable, buildPairPosGlyphs, buildPairPosGlyphsSubtable, buildSinglePos, buildSinglePosSubtable
+   :member-order: bysource
+
+----------------------------
+GDEF Table Subtable Builders
+----------------------------
+
+These functions build subtables for elements of the ``GDEF`` table.
+
+.. automodule:: fontTools.otlLib.builder
+   :members: buildAttachList, buildLigCaretList, buildMarkGlyphSetsDef
+   :member-order: bysource
+
+------------------
+STAT Table Builder
+------------------
+
+.. automodule:: fontTools.otlLib.builder
+   :members: buildStatTable
+   :member-order: bysource