[docs] Add note for default_value hack only working with positional args

@jchidley noted in #11 that the default_value hack for omitting the "-" st
ring doesn't work with optional args. Updated the docs to highlight this.

Also updated usages of #[clap(...)] to #[arg(...)] as that seems to be the
new macro name for clap attributes

Author: thepacketgeek

README.md

@@ -103,7 +103,9 @@ $ .example myfile.txt
 ```
 
 ## Reading from Stdin without special characters
-When using [`MaybeStdin`] or [`FileOrStdin`], you can allow your users to omit the "-" character to read from `stdin` by providing a `default_value` to clap. This works with positional and optional args:
+When using [`MaybeStdin`] or [`FileOrStdin`], you can allow your users to omit the "-" character to read from `stdin` by providing a `default_value` to clap.
+
+**NOTE:** This only works with positional args, since clap requires optional args (E.g. using #[arg(long, short)]) to have a value to parse.
 
 ```rust,no_run
 use clap::Parser;
@@ -112,7 +114,7 @@ use clap_stdin::FileOrStdin;
 
 #[derive(Debug, Parser)]
 struct Args {
-    #[clap(default_value = "-")]
+    #[arg(default_value = "-")]
     input: FileOrStdin,
 }
 

examples/parse_with_serde.rs

@@ -39,7 +39,7 @@ impl FromStr for User {
 #[derive(Debug, Parser)]
 struct Args {
     /// Parsed user from json, provided via a filepath (or leave blank to read from stdin)
-    #[clap(default_value = "-")]
+    #[arg(default_value = "-")]
     user: FileOrStdin<User>,
 }
 

tests/fixtures/file_or_stdin_optional_arg.rs

@@ -5,7 +5,7 @@ use clap_stdin::FileOrStdin;
 #[derive(Debug, Parser)]
 struct Args {
     first: String,
-    #[clap(short, long)]
+    #[arg(short, long)]
     second: Option<FileOrStdin<u32>>,
 }
 

tests/fixtures/file_or_stdin_positional_arg.rs

@@ -4,9 +4,9 @@ use clap_stdin::FileOrStdin;
 
 #[derive(Debug, Parser)]
 struct Args {
-    #[clap(default_value = "-")]
+    #[arg(default_value = "-")]
     first: FileOrStdin,
-    #[clap(short, long)]
+    #[arg(short, long)]
     second: Option<String>,
 }
 

tests/fixtures/maybe_stdin_optional_arg.rs

@@ -5,7 +5,7 @@ use clap_stdin::MaybeStdin;
 #[derive(Debug, Parser)]
 struct Args {
     first: String,
-    #[clap(short, long)]
+    #[arg(short, long)]
     second: Option<MaybeStdin<u32>>,
 }
 

tests/fixtures/maybe_stdin_positional_arg.rs

@@ -5,7 +5,7 @@ use clap_stdin::MaybeStdin;
 #[derive(Debug, Parser)]
 struct Args {
     first: MaybeStdin<String>,
-    #[clap(short, long)]
+    #[arg(short, long)]
     second: Option<String>,
 }