Help: Document special cases for if(IS_ABSOLUTE)

The meaning of "absolute path" was previously assumed knowledge,
but a number of special cases were left unspecified. The way some of
these are handled differs to the way that cmake_path(IS_ABSOLUTE)
works, so document those special cases so that the differing behavior
between these two commands is clearly defined.

Help/command/if.rst

@@ -153,7 +153,16 @@ File Operations
  only for full paths.
 
 ``if(IS_ABSOLUTE path)``
- True if the given path is an absolute path.
+ True if the given path is an absolute path.  Note the following special
+ cases:
+
+ * An empty ``path`` evaluates to false.
+ * On Windows hosts, any ``path`` that begins with a drive letter and colon
+   (e.g. ``C:``), a forward slash or a backslash will evaluate to true.
+   This means a path like ``C:no\base\dir`` will evaluate to true, even
+   though the non-drive part of the path is relative.
+ * On non-Windows hosts, any ``path`` that begins with a tilde (``~``)
+   evaluates to true.
 
 Comparisons
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