add doc comments for VError and VInterpolate

cplusplus/include/vips/VError8.h

@@ -43,12 +43,27 @@ class VIPS_CPLUSPLUS_API VError : public std::exception {
 	std::string _what;
 
 public:
+	/**
+	 * Construct a VError, setting the error message.
+	 */
 	VError( const std::string &what ) : _what( what ) {}
+
+	/** 
+	 * Construct a VError, fetching the error message from the libvips
+	 * error buffer.
+	 */
 	VError() : _what( vips_error_buffer() ) {}
+
 	virtual ~VError() throw() {}
 
-	// Extract string
+	/**
+	 * Get a reference to the underlying C string.
+	 */
 	virtual const char *what() const throw() { return _what.c_str(); }
+
+	/**
+	 * Print the error message to a stream.
+	 */
 	void ostream_print( std::ostream & ) const;
 };
 

cplusplus/include/vips/VInterpolate8.h

@@ -37,14 +37,25 @@ VIPS_NAMESPACE_START
 class VInterpolate : VObject
 {
 public:
+	/**
+	 * Create a VInterpolate that wraps a VipsInterpolate object. If steal
+	 * is STEAL, then this VInterpolate takes over ownership of the libvips
+	 * object and will automatically unref it.
+	 */
 	VInterpolate( VipsInterpolate *interpolate, VSteal steal = STEAL ) : 
 		VObject( (VipsObject *) interpolate, steal )
 	{
 	}
 
+	/**
+	 * Create a VInterpolate from a name, for example `"bicubic"`.
+	 */
 	static 
 	VInterpolate new_from_name( const char *name, VOption *options = 0 );
 
+	/**
+	 * Get a pointer to the underlying VipsInterpolate object.
+	 */
 	VipsInterpolate *
 	get_interpolate() const
 	{