Clarify namespace version prefix support in docs

Updated XML comments and code documentation in NamespaceParser.cs to specify that API version parsing supports 'v', 'V', or '_' as valid namespace prefixes. Added remarks and examples explaining the use of the underscore prefix, especially for folder names starting with numbers. Improved TryParse parameter documentation to reflect these changes.

Author: xavierjohn

src/Abstractions/src/Asp.Versioning.Abstractions/NamespaceParser.cs

@@ -17,6 +17,11 @@ namespace Asp.Versioning;
 /// <summary>
 /// Represents API version parser from a type namespace.
 /// </summary>
+/// <remarks>
+/// The namespace identifier can use 'v', 'V', or '_' as a prefix. The '_' prefix is useful when
+/// a folder starts with a number because Visual Studio automatically prefixes it with an underscore.
+/// For example, <c>Contoso.Api._2018_04_01.Controllers</c> is equivalent to <c>Contoso.Api.v2018_04_01.Controllers</c>.
+/// </remarks>
 public class NamespaceParser
 {
     private const string CompactDateFormat = "yyyyMMdd";
@@ -142,7 +147,8 @@ public IReadOnlyList<ApiVersion> Parse( Type type )
     /// <summary>
     /// Attempts to parse an API version from the specified namespace identifier.
     /// </summary>
-    /// <param name="identifier">The namespace identifier to parse.</param>
+    /// <param name="identifier">The namespace identifier to parse. The identifier must start with
+    /// 'v', 'V', or '_' followed by the version components.</param>
     /// <param name="apiVersion">The parsed <see cref="ApiVersion">API version</see> or <c>null</c>.</param>
     /// <returns>True if parsing is successful; otherwise, false.</returns>
     protected virtual bool TryParse( Text identifier, out ApiVersion? apiVersion )
@@ -164,7 +170,12 @@ protected virtual bool TryParse( Text identifier, out ApiVersion? apiVersion )
         // - v2_0_Beta
         // - v20180401
         // - v2018_04_01_1_1_Beta
+        // - _1
+        // - _1_1
+        // - _20180401
         // - _2018_04_01
+        // - _2018_04_01_Beta
+        // - _2018_04_01_1_0_Beta
         var ch = identifier[0];
 
         if ( ch != 'v' && ch != 'V' && ch != '_' )