Documentation corrections

Corrected some spelling errors and added / correceted some references
Handlingh of -- by \c and <code> results in - adjusted documentation

Adjusted documentation so that -- in \c and <code> are shown as -- by placing the parts in ` (backtick)
Use hook arrow for hyphens in symbol names in the LaTeX output.
Fix to VHDL scanner.

The following code should produce two groups, each containing one page, within the same documentation scope,
currently it nests these groups and pages, resulting in incorrect documentation.

--! \defgroup group1 Group 1
--! \{
--!   @brief Group 1
--! \}

--! \ingroup group1
--! \{
--! \page group1_page1 Page 1
--! \verbatim
--!  Some Page 1 text
--! \endverbatim
--! \}

--! \defgroup group2 Group 2
--! \{
--!   @brief Group 2
--! \}

--! \ingroup group2
--! \{
--! \page group2_page1 Page 1
--! \verbatim
--!  Some Page 1 text
--! \endverbatim
--! \}

Similar C code to demonstrate the correct output.

//! \defgroup group1 Group 1
//! \{
//!   @brief Group 1
//! \}

//! \ingroup group1
//! \{
//! \page group1_page1 Page 1
//! \verbatim
//!  Some Page 1 text
//! \endverbatim
//! \}

//! \defgroup group2 Group 2
//! \{
//!   @brief Group 2
//! \}

//! \ingroup group2
//! \{
//! \page group2_page1 Page 1
//! \verbatim
//!  Some Page 1 text
//! \endverbatim
//! \}
Fixed potential crash caused by overloading the variadic warn function
Merge branch 'master' of github.com:doxygen/doxygen
Merge pull request #99 from CraigNoble/master

Fix to VHDL scanner.
Merge pull request #89 from albert-github/feature/docu_update

Documentation corrections