docs: Clarify corner caseses with default values

This is meant to lower the chance of confusion with cases like clap-rs#2714 and clap-rs#1586.

This is not meant to be exhaustive, looked at the mentioned cases in
that issue and pattern matched on other ones mentioning "is present".

src/build/arg/mod.rs

@@ -1045,6 +1045,8 @@ impl<'help> Arg<'help> {
     ///
     /// **NOTE** [`Arg::exclusive(true)`] allows specifying an argument which conflicts with every other argument.
     ///
+    /// **NOTE:** An argument is considered present when there is a [`Arg::default_value`]
+    ///
     /// # Examples
     ///
     /// ```rust
@@ -1095,6 +1097,8 @@ impl<'help> Arg<'help> {
     ///
     /// **NOTE:** [`Arg::exclusive(true)`] allows specifying an argument which conflicts with every other argument.
     ///
+    /// **NOTE:** An argument is considered present when there is a [`Arg::default_value`]
+    ///
     /// # Examples
     ///
     /// ```rust
@@ -1328,6 +1332,8 @@ impl<'help> Arg<'help> {
     ///
     /// **NOTE:** [Conflicting] rules and [override] rules take precedence over being required
     ///
+    /// **NOTE:** This argument is considered present when there is a [`Arg::default_value`]
+    ///
     /// # Examples
     ///
     /// ```rust
@@ -1389,6 +1395,8 @@ impl<'help> Arg<'help> {
     /// if this arg (`self`) is present and its value equals to `val`.
     /// If it does, `another_arg` will be marked as required.
     ///
+    /// **NOTE:** This argument is considered present when there is a [`Arg::default_value`]
+    ///
     /// # Examples
     ///
     /// ```rust
@@ -1510,6 +1518,8 @@ impl<'help> Arg<'help> {
     /// Allows specifying that this argument is [required] only if the specified
     /// `arg` is present at runtime and its value equals `val`.
     ///
+    /// **NOTE:** This argument is considered present when there is a [`Arg::default_value`]
+    ///
     /// # Examples
     ///
     /// ```rust
@@ -1786,6 +1796,8 @@ impl<'help> Arg<'help> {
     /// **NOTE:** [Conflicting] rules and [override] rules take precedence over being required
     /// by default.
     ///
+    /// **NOTE:** This argument is considered present when there is a [`Arg::default_value`]
+    ///
     /// # Examples
     ///
     /// ```rust

src/build/arg_group.rs

@@ -249,6 +249,8 @@ impl<'help> ArgGroup<'help> {
     /// Use of more than one arg is an error." Vice setting `ArgGroup::multiple(true)` which
     /// states, '*At least* one arg from this group must be used. Using multiple is OK."
     ///
+    /// **NOTE:** An argument is considered present when there is a [`Arg::default_value`]
+    ///
     /// # Examples
     ///
     /// ```rust
@@ -284,6 +286,8 @@ impl<'help> ArgGroup<'help> {
     ///
     /// **NOTE:** The name provided may be an argument, or group name
     ///
+    /// **NOTE:** An argument is considered present when there is a [`Arg::default_value`]
+    ///
     /// # Examples
     ///
     /// ```rust
@@ -319,6 +323,8 @@ impl<'help> ArgGroup<'help> {
     ///
     /// **NOTE:** The names provided may be an argument, or group name
     ///
+    /// **NOTE:** An argument is considered present when there is a [`Arg::default_value`]
+    ///
     /// # Examples
     ///
     /// ```rust
@@ -357,6 +363,8 @@ impl<'help> ArgGroup<'help> {
     ///
     /// **NOTE:** The name provided may be an argument, or group name
     ///
+    /// **NOTE:** An argument is considered present when there is a [`Arg::default_value`]
+    ///
     /// # Examples
     ///
     /// ```rust
@@ -389,6 +397,8 @@ impl<'help> ArgGroup<'help> {
     ///
     /// **NOTE:** The names provided may be an argument, or group name
     ///
+    /// **NOTE:** An argument is considered present when there is a [`Arg::default_value`]
+    ///
     /// # Examples
     ///
     /// ```rust