Correct conan build path arg description

Change conan build argument description for "path" to indicate it is
only for conanfile.py and explicitly state that it does not work with
conanfile.txt.

Resolves conan-io#13198

conan/cli/commands/build.py

@@ -15,9 +15,9 @@ def build(conan_api, parser, *args):
     Install dependencies and call the build() method.
     """
     parser.add_argument("path", nargs="?",
-                        help="Path to a folder containing a recipe (conanfile.py "
-                             "or conanfile.txt) or to a recipe file. e.g., "
-                             "./my_project/conanfile.txt.")
+                        help='Path to a python-based recipe file or a folder '
+                             'containing a conanfile.py recipe. conanfile.txt '
+                             'cannot be used with conan build.')
     add_reference_args(parser)
     # TODO: Missing --build-require argument and management
     parser.add_argument("-of", "--output-folder",