small doc improvements

doc/Principles.md

@@ -52,7 +52,7 @@ In all of these modes the library client can specify arguments that indicate how
 
 ### SGFC messages
 
-When SGFC reads or writes SGF data it often generates one or more messages. Every message has an ID, and every message ID is an indicator for what the message is about. The SGFC documentation lists all messages with their IDs and meanings.
+When SGFC reads or writes SGF data it often generates one or more messages. Every message has an ID, and every message ID is an indicator for what the message is about. libsgfc++ defines the enumeration `SgfcMessageID` with all possible message IDs accompanied by sparse documentation. To see the full documentation consult the SGFC documentation where all messages are listed with their IDs and meanings.
 
 The library processes the message data generated by SGFC and makes it available to the library client in the form of a collection of `ISgfcMessage` objects. These objects provide the data in a structured form so that the library client can programmatically evaluate the message content and possibly handle certain warnings or errors.
 

doc/SgfcNotes.md

@@ -27,7 +27,7 @@ When reading SimpleText or Text property values, SGFC follows the SGF standard r
 - Text property values: SGFC preserves unescaped (= hard) line breaks. SGFC removes escaped (= soft) line breaks (including the escape character). libsgfc++ never gets to see escaped line breaks.
 - SimpleText property values: SGFC converts unescaped line breaks into a single space character. SGFC removes escaped line breaks (including the escape character). libsgfc++ never gets to see escaped line breaks.
 
-When writing SimpleText or Text property values, SGFC preserves unescaped line breaks and generates escaped line breaks as needed.
+When writing SimpleText or Text property values, SGFC preserves unescaped line breaks and generates escaped line breaks as it sees fit (cf. `-L` and `-t` command line options).
 
 ## Pass moves for Go games
 

include/ISgfcGameInfo.h

@@ -50,15 +50,23 @@ namespace LibSgfcPlusPlus
   /// properties (SgfcPropertyCategory::GameInfo) found in the root node and the
   /// game info node, respectively, at the time the query is made.
   ///
+  /// @note It is not possible to distinguish between a property that was not
+  /// present in the root or game info node, and a property that was present
+  /// in the root or game info but had the default property value.
+  ///
   /// Alternatively a new but empty ISgfcGameInfo object can be created via
   /// SgfcPlusPlusFactory and then populated with data as the library client
-  /// sees fit.
+  /// sees fit. The empty ISgfcGameInfo object is initialized with default
+  /// values.
   ///
   /// The values that an ISgfcGameInfo object holds can be written to an
   /// ISgfcGame or ISgfcNode. The values are converted to properties and
   /// property values that are then stored in the root node and the game info
   /// node, respectively. Properties and property values that already exist at
-  /// that time are overwritten.
+  /// that time are either overwritten, or removed if the value to be written
+  /// is the default value.
+  ///
+  /// @note It is not possible to write a property with its default value.
   ///
   /// Root property values are read-only - they must be provided at the time the
   /// ISgfcGameInfo object is constructed and cannot be changed later on. The