patch 8.1.0314: build failure without the +eval feature

Problem:    Build failure without the +eval feature. (Brenton Horne)
Solution:   Add #ifdef.  Also add the "dirty" item.
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index a9d0dc0..2e5bf7f 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -8013,10 +8013,12 @@
 				file
 			mtime	last modification time in seconds
 			inode	Optional: INODE number of the file
+			dirty	1 if file was modified, 0 if not
 		In case of failure an "error" item is added with the reason:
 			Cannot open file: file not found or in accessible
 			Cannot read file: cannot read first block
-			magic number mismatch: info in first block is invalid
+			Not a swap file: does not contain correct block ID
+			Magic number mismatch: Info in first block is invalid
 
 synID({lnum}, {col}, {trans})				*synID()*
 		The result is a Number, which is the syntax ID at the position