Document about size pragma (#21794)

* Document about size pragma

* Fix typos

* Fix manual.md

* Update doc/manual.md

---------

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

doc/manual.md

@@ -7485,6 +7485,37 @@ generates:
   ```
 
 
+size pragma
+-----------
+Nim automatically determines the size of an enum.
+But when wrapping a C enum type, it needs to be of a specific size.
+The `size pragma` allows specifying the size of the enum type.
+
+  ```Nim
+  type
+    EventType* {.size: sizeof(uint32).} = enum
+      QuitEvent,
+      AppTerminating,
+      AppLowMemory
+
+  doAssert sizeof(EventType) == sizeof(uint32)
+  ```
+
+The `size pragma` can also specify the size of an `importc` incomplete object type
+so that one can get the size of it at compile time even if it was declared without fields.
+
+  ```Nim
+    type
+      AtomicFlag* {.importc: "atomic_flag", header: "<stdatomic.h>", size: 1.} = object
+
+    static:
+      # if AtomicFlag didn't have the size pragma, this code would result in a compile time error.
+      echo sizeof(AtomicFlag)
+  ```
+
+The `size pragma` accepts only the values 1, 2, 4 or 8.
+
+
 Align pragma
 ------------