C#: Update change notes.

Author: calumgrant

change-notes/1.20/analysis-csharp.md

@@ -16,6 +16,7 @@
 | Dereferenced variable may be null (cs/dereferenced-value-may-be-null) | Improved results | The query has been rewritten from scratch, and the analysis is now based on static single assignment (SSA) forms. The query is now enabled by default in LGTM. |
 | SQL query built from user-controlled sources (cs/sql-injection), Improper control of generation of code (cs/code-injection), Uncontrolled format string (cs/uncontrolled-format-string), Clear text storage of sensitive information (cs/cleartext-storage-of-sensitive-information), Exposure of private information (cs/exposure-of-sensitive-information) | More results | Data sources have been added from user controls in `System.Windows.Forms`. |
 | Use of default ToString() (cs/call-to-object-tostring) | Fewer false positives | Results have been removed for `char` arrays passed to `StringBuilder.Append()`, which were incorrectly marked as using `ToString`. |
+| Use of default ToString() (cs/call-to-object-tostring) | Fewer results | Results have been removed when the object is an interface or an abstract class. |
 
 ## Changes to code extraction