Clarify doc on absolute path imports (#1597)

* Clarify doc on absolute path imports

The current documentation caused confusion with the term "absolute
path" in import statements in combination with the search path
directories. An example should avoid this confusion.

* Update doc/language.md

Co-authored-by: Kenton Varda <kenton@sandstorm.io>
diff --git a/doc/language.md b/doc/language.md
index a0bfad9..83e144e 100644
--- a/doc/language.md
+++ b/doc/language.md
@@ -581,7 +581,9 @@
 
 The above imports specify relative paths.  If the path begins with a `/`, it is absolute -- in
 this case, the `capnp` tool searches for the file in each of the search path directories specified
-with `-I`.
+with `-I`, appending the path you specify to the path given to the `-I` flag. So, for example,
+if you ran `capnp` with `-Ifoo/bar`, and the import statement is `import "/baz/qux.capnp"`, then
+the compiler would open the file `foo/bar/baz/qux.capnp`.
 
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