Fixed doxygen warnings reported by recent doxygen versions.

Changes in Doxyfile.in:
- upgraded config to doxygen 1.8.20,
  while commenting new options not understood by doxygen 1.8.13,
  for backward compatibility with 1.8.13.
- removed the obsolete TCL_SUBST option
- set EXTRACT_ANON_NSPACES to YES,
  which resolves many warnings related to code using anonymous namespace.
- excluded test code from the doxygen build.

Changes in the source code in general:
- removed duplicated page PAGE_COMPONENTS_REGISTRY
- fixed many doc duplication between declarations and definitions
- fixed some typos in doc
- fixed formatting of retval for tuples
- fixed unbalanced grouping commands

Tested with doxygen 1.8.20.

Approved by: Chris Powers <chris.powers@oracle.com>