Partial fix #14227 (Add checker documentation for truncLongCastAssignment and truncLongCastReturn) [skip ci] (danmar#7979)

Author: danmar

man/checkers/truncLongCast.md

@@ -0,0 +1,50 @@
+# truncLongCastAssignment and truncLongCastReturn
+
+**Message**: int result is assigned to long variable. If the variable is long to avoid loss of information, then you have loss of information.<br/>
+**Category**: Type Safety<br/>
+**Severity**: Style<br/>
+**Language**: C/C++
+
+## Description
+
+This checker warns when a calculation has type 'int' and it could potentially overflow that and the result is implicitly or explicitly converted to a larger
+integer type after the loss of information has already occured.
+
+## Motivation
+
+The motivation of this checker is to catch bugs. Unintentional loss of information.
+
+## How to fix
+
+You can fix these warnings by:
+1. Add explicit cast to avoid loss of information
+2. Explicitly truncate the result
+3. Change type of assigned variable
+
+Before:
+```cpp
+void foo(int32_t y) {
+    int64_t x = y * y; // <- warning
+}
+```
+
+After (explicit cast):
+```cpp
+void foo(int32_t y) {
+    int64_t x = (int64_t)y * y; // <- 64-bit multiplication
+}
+```
+
+After (explicitly truncate the result):
+```cpp
+void foo(int32_t y) {
+    int64_t x = (int32_t)(y * y); // redundant cast makes it explicit that your intention is to truncate the result to 32-bit
+}
+```
+
+After (change type of assigned variable):
+```cpp
+void foo(int32_t y) {
+    int32_t x = y * y;
+}
+```