Updated SC2148 (markdown)

SC2148.md

@@ -1,20 +1,29 @@
-##  Shebang (#!) missing. Assuming Bash.
+##  Tips depend on target shell and yours is unknown. Add a shebang.
 
 ### Problematic code:
 
-    echo "Hello World"
+    echo "$RANDOM"   # Does this work
 
 ### Correct code:
 
     #!/bin/sh
-    echo "Hello World"
+    echo "$RANDOM"  # Unsupported in sh. Produces warning.
+
+or
+
+    #!/bin/bash
+    echo "$RANDOM"  # Supported in bash. No warnings.
 
 ### Rationale:
 
-ShellCheck warns about different things for different shells. It uses the shebang to determine which shell to warn for. For the command line client, this can be overridden with `-s`, e.g. `shellcheck -s sh file`. 
+Different shells support different features. To give effective advice, ShellCheck needs to know which shell your script is going to run on. You will get a different numbers of warnings about different things depending on your target shell.
 
-If you don't specify the shell type in any way, ShellCheck assumes a default and gives this message.
+ShellCheck normally determines your target shell from the shebang (having e.g. `#!/bin/sh` as the first line). The shell can also be specified from the CLI with `-s`, e.g. `shellcheck -s sh file`. 
+
+If you don't specify shebang nor `-s`, ShellCheck gives this message and proceeds with some default (`bash`).
+
+Note that this error can not be ignored with a [[directive]]. It is not a suggestion to improve your script, but a warning that ShellCheck lacks information it needs to be helpful. 
 
 ### Exceptions
 
-None. Just specify the shell type, either in a shebang or as a cli flag.
+None. Please either add a shebang or use `-s`.