add explaination about @execute field with custom language

doc/frontend.md

@@ -875,8 +875,7 @@ As described above the `@execute` key can be written in any language as long as
 
 ```
     @build = custom
-    @execute = (
-        #!/usr/bin/bash
+    @execute = (#!/usr/bin/bash
         echo "This script displays available services"
         for i in $(ls %%service_system%%); do
             echo "daemon : ${i} is available"
@@ -888,10 +887,68 @@ This is an unnecessary example but it shows how to construct this use case. The
 
 ```
     #!/usr/bin/bash
-    echo "This script displays available services"
-    for i in $(ls %%service_system%%); do
-        echo "daemon : ${i} is available"
-    done
+        echo "This script displays available services"
+        for i in $(ls %%service_system%%); do
+            echo "daemon : ${i} is available"
+        done
+```
+
+The parser creates an exact copy of what it finds between `(` and `)`. This means that regardless of the character found, it retains it. For instance, if you write
+```
+@execute = (
+#!/bin/bash echo hello world!
+)
+```
+the final result will contain a newline at the very beginning corresponding to the newline found between `(` and the definition of the shebang `#!/bin/bash`. In this case, when executing the service, you'll encounter an ***Exec format error*** because the very first line corresponds to a newline instead of the shebang declaration.
+
+To avoid this issue, ALWAYS declare the shebang of your script directly after `(` and without any spaces, tabs, newlines, etc. For example,
+```
+@execute = (#!/bin/bash
+echo hello world!
+)
+```
+
+When using the `@execute` field with your script language, ensure that the `)` character is not followed by a new line that begins with `@`, `#[A-Z]`, or `#`@ at the very start of the line. This is to ensure validity. For example, the following code snippets are invalid:
+
+```
+@execute = (#!/bin/sh
+
+option="${1}"
+case ${option} in
+   -f) FILE="${2}"
+#@FILE: param for echo command
+      echo "File name is $FILE"
+      ;;
+   -d) DIR="${2}"
+      echo "Dir name is $DIR"
+      ;;
+   *)
+      echo "`basename ${0}`:usage: [-f file] | [-d directory]"
+      exit 1 # Command to come out of the program with status 1
+      ;;
+esac
+```
+
+
+The parser looks for the sequence `@`, `#[A-Z]`, or `#@` to validate the last `)` character in the `@execute` field. To correct this, ensure that the sequence doesn’t start at the beginning of the line following the `)` character. The corrected examples show this by adding a tab or space in front of `#@FILE`.
+
+```
+@execute = (#!/bin/sh
+option="${1}"
+case ${option} in
+   -f) FILE="${2}"
+    #@FILE: param for echo command
+      echo "File name is $FILE"
+      ;;
+   -d) DIR="${2}"
+      echo "Dir name is $DIR"
+      ;;
+   *)
+      echo "`basename ${0}`:usage: [-f file] | [-d directory]"
+      exit 1 # Command to come out of the program with status 1
+      ;;
+esac
+)
 ```
 
 When using this sort of custom function `@runas` has **no effect**. You **must** define with care what you want to happen in a *custom* case.