[Diagnostics] Fix typos and flag names

Author: DmT021

include/swift/AST/DiagnosticEngine.h

@@ -868,7 +868,7 @@ namespace swift {
     /// Don't emit any remarks
     bool suppressRemarks = false;
 
-    /// Treat these warnings as errors. Indicies here corespond to DiagID enum
+    /// Treat these warnings as errors. Indices here correspond to DiagID enum
     llvm::BitVector warningsAsErrors;
 
     /// Whether a fatal error has occurred
@@ -1136,7 +1136,7 @@ namespace swift {
     }
 
     /// Apply rules specifing what warnings should or shouldn't be treated as
-    /// errors. For group rules the string is either a group name defined by
+    /// errors. For group rules the string is a group name defined by
     /// DiagnosticGroups.def
     /// Rules are applied in order they appear in the vector.
     /// In case the vector contains rules affecting the same diagnostic ID

userdocs/diagnostic_groups/availability_deprecated.md

@@ -25,6 +25,6 @@ The `availability_deprecated` group covers the following warnings:
 ## Usage Example
 
 ```sh
-swiftc -warning-as-error availability_deprecated file.swift
-swiftc -warnings-as-errors -no-warning-as-error availability_deprecated file.swift
+swiftc -Werror availability_deprecated file.swift
+swiftc -warnings-as-errors -Wwarning availability_deprecated file.swift
 ```

userdocs/diagnostic_groups/deprecated.md

@@ -7,6 +7,6 @@ The deprecated group is a supergroup designed to manage all kinds of warnings re
 ## Usage Example
 
 ```sh
-swiftc -warning-as-error deprecated file.swift
-swiftc -warnings-as-errors -no-warning-as-error deprecated file.swift
+swiftc -Werror deprecated file.swift
+swiftc -warnings-as-errors -Wwarning deprecated file.swift
 ```

userdocs/diagnostic_groups/unknown_warning_group.md

@@ -3,13 +3,13 @@
 The `unknown_warning_group` diagnostic group addresses warnings related to the specification of unrecognized warning groups in compilation flags.
 
 ```sh
-swiftc -warning-as-error non_existing_group file.swift
+swiftc -Werror non_existing_group file.swift
 <unknown>:0: warning: unknown warning group: 'non_existing_group'
 ```
 
 Such warnings are emitted after the behavior for all specified warning groups has been processed, which means their behavior can also be specified. For example:
 
 ```sh
-swiftc -warning-as-error unknown_warning_group -warning-as-error non_existing_group file.swift
+swiftc -Werror unknown_warning_group -Werror non_existing_group file.swift
 <unknown>:0: error: unknown warning group: 'non_existing_group'
 ```