<host_version> syntax (#3223)

* explained host_version

* Update reference/conanfile/methods/build_requirements.rst

Co-authored-by: Luis Caro Campos <3535649+jcar87@users.noreply.github.com>

* Suggestions

* Update reference/conanfile/methods/build_requirements.rst

Co-authored-by: Rubén Rincón Blanco <git@rinconblanco.es>

* Update reference/conanfile/methods/build_requirements.rst

Co-authored-by: Rubén Rincón Blanco <git@rinconblanco.es>

* Update reference/conanfile/methods/build_requirements.rst

Co-authored-by: Rubén Rincón Blanco <git@rinconblanco.es>

---------

Co-authored-by: Luis Caro Campos <3535649+jcar87@users.noreply.github.com>
Co-authored-by: Rubén Rincón Blanco <git@rinconblanco.es>

reference/conanfile/methods/build_requirements.rst

@@ -27,6 +27,48 @@ The ``tool_requires`` is equivalent to ``requires()`` with the following traits:
 - ``headers=False`` A tool requirement does not have headers.
 - ``libs=False``: A tool requirement does not have libraries to be linked by the consumer (if it had libraries they would be in the "build" context and could be incompatible with the "host" context of the consumer package). 
 
+<host_version>
+**************
+
+.. include:: ../../../common/experimental_warning.inc
+
+This syntax is useful when you're using the same package recipe as a *requires* and as a *tool_requires* and you want to avoid
+conflicting downstream if any user decides to override the original *requires* version in the *host* context, i.e., the user could end up with
+two different versions in the host and build contexts of the same dependency.
+
+In a nutshell, the ``<host_version>`` specifier allows us to ensure that the version resolved for the *tool_requires*
+always matches the one for the host requirement.
+
+For instance, let's show a simple recipe using *protobuf*:
+
+.. code-block:: python
+
+    from conan import ConanFile
+
+    class mylibRecipe(ConanFile):
+        name = "mylib"
+        version = "0.1"
+        def requirements(self):
+            self.requires("protobuf/3.18.1")
+        def build_requirements(self):
+            self.tool_requires("protobuf/<host_version>")
+
+Then, if any user wants to use *mylib/0.1*, but another version of *protobuf*, there shouldn't be any problems overriding it:
+
+.. code-block:: python
+
+    from conan import ConanFile
+
+    class myappRecipe(ConanFile):
+        name = "myapp"
+        version = "0.1"
+        def requirements(self):
+            self.requires("mylib/0.1")
+            self.requires("protobuf/3.21.9", override=True)
+
+The ``<host_version>`` defined upstream is ensuring that the host and build contexts are using the same version of that requirement.
+
+
 test_requires
 -------------