expr: update help to markdown syntax
## About
-Print the value of EXPRESSION to standard output
+Print the value of `EXPRESSION` to standard output
## Usage
```
@@ -12,8 +12,10 @@ expr [OPTIONS]
## After help
-Print the value of EXPRESSION to standard output. A blank line below
-separates increasing precedence groups. EXPRESSION may be:
+Print the value of `EXPRESSION` to standard output. A blank line below
+separates increasing precedence groups.
+
+`EXPRESSION` may be:
ARG1 | ARG2 ARG1 if it is neither null nor 0, otherwise ARG2
@@ -49,11 +51,11 @@ Comparisons are arithmetic if both ARGs are numbers, else lexicographical.
Pattern matches return the string matched between \( and \) or null; if
\( and \) are not used, they return the number of characters matched or 0.
-Exit status is 0 if EXPRESSION is neither null nor 0, 1 if EXPRESSION is null
-or 0, 2 if EXPRESSION is syntactically invalid, and 3 if an error occurred.
+Exit status is `0` if `EXPRESSION` is neither null nor `0`, `1` if `EXPRESSION` is null
+or `0`, `2` if `EXPRESSION` is syntactically invalid, and `3` if an error occurred.
Environment variables:
- * EXPR_DEBUG_TOKENS=1 dump expression's tokens
- * EXPR_DEBUG_RPN=1 dump expression represented in reverse polish notation
- * EXPR_DEBUG_SYA_STEP=1 dump each parser step
- * EXPR_DEBUG_AST=1 dump expression represented abstract syntax tree"
\ No newline at end of file
+ - `EXPR_DEBUG_TOKENS=1`: dump expression's tokens
+ - `EXPR_DEBUG_RPN=1`: dump expression represented in reverse polish notation
+ - `EXPR_DEBUG_SYA_STEP=1`: dump each parser step
+ - `EXPR_DEBUG_AST=1`: dump expression represented abstract syntax tree
\ No newline at end of file
