Updated SC1090 (markdown)

SC1090.md

@@ -1,20 +1,20 @@
-## This source will be skipped since it's not constant.
+## Can't follow non-constant source. Use a directive to specify location.
 
 ### Problematic code:
 
-    source "$1/lib.sh"
+    . "$(find_install_dir)/lib.sh"
 
 ### Correct code:
 
-    # shellcheck disable=SC1090
-    source "$1/lib.sh"
+    # shellcheck source=src/lib.sh
+    . "$(find_install_dir)/lib.sh"
 
 ### Rationale:
 
-ShellCheck is not able to include sourced files from non-constant paths.
+ShellCheck is not able to include sourced files from paths that are determined at runtime. The file will not be read, potentially resulting in warnings about unassigned variables and similar.
 
-Instead, be aware that anything in the sourced file will be ignored, and then disable the message.
+Use a [[Directive]] to point shellcheck to a fixed location it can read instead. 
 
 ### Exceptions:
 
-Ignore the message with a [[directive]].
+If you don't care that ShellCheck is unable to account for the file, specify `# shellcheck source=/dev/null`.