Add description of labels to source format chapter

The source format chapter was missing details about the characters that
are valid for labels and what labels are for. Add a paragraph to cover
those details.

Signed-off-by: Grant Likely <[email protected]>

source/source-language.rst

@@ -22,6 +22,34 @@ additional files.
 
     /include/ "FILE"
 
+Labels
+------
+
+The source format allows labels to be attached to any node or property value in the device tree.
+Phandle and path references can be automatically generated by referencing a label instead of
+explicitly specifying a phandle value or the full path to a node.
+Labels are only used in the devicetree source format and are not encoded into the DTB binary.
+
+A label shall be between 1 to 31 characters in length,
+be composed only of the characters in the set :numref:`label-characters`,
+and must not start with a number.
+
+Labels are created by appending a colon (':') to the label name.
+References are created by prefixing the label name with an ampersand ('&').
+
+.. tabularcolumns:: | c p{8cm} |
+.. _label-characters:
+.. table:: Valid characters for DTS labels
+
+   ========= ================
+   Character Description
+   ========= ================
+   ``0-9``   digit
+   ``a-z``   lowercase letter
+   ``A-Z``   uppercase letter
+   ``_``     underscore
+   ========= ================
+
 Node and property definitions
 -----------------------------