Improve effeciency of TextSplitter.split_documents, iterate once (#5111)

# Improve TextSplitter.split_documents, collect page_content and
metadata in one iteration

## Who can review?

Community members can review the PR once tests pass. Tag
maintainers/contributors who might be interested:

@eyurtsev In the case where documents is a generator that can only be
iterated once making this change is a huge help. Otherwise a silent
issue happens where metadata is empty for all documents when documents
is a generator. So we expand the argument from `List[Document]` to
`Union[Iterable[Document], Sequence[Document]]`

---------

Co-authored-by: Steven Tartakovsky <tartakovsky.developer@gmail.com>

tests/unit_tests/test_text_splitter.py

@@ -146,3 +146,25 @@ Bye!\n\n-H."""
         "Bye!\n\n-H.",
     ]
     assert output == expected_output
+
+
+def test_split_documents() -> None:
+    """Test split_documents."""
+    splitter = CharacterTextSplitter(separator="", chunk_size=1, chunk_overlap=0)
+    docs = [
+        Document(page_content="foo", metadata={"source": "1"}),
+        Document(page_content="bar", metadata={"source": "2"}),
+        Document(page_content="baz", metadata={"source": "1"}),
+    ]
+    expected_output = [
+        Document(page_content="f", metadata={"source": "1"}),
+        Document(page_content="o", metadata={"source": "1"}),
+        Document(page_content="o", metadata={"source": "1"}),
+        Document(page_content="b", metadata={"source": "2"}),
+        Document(page_content="a", metadata={"source": "2"}),
+        Document(page_content="r", metadata={"source": "2"}),
+        Document(page_content="b", metadata={"source": "1"}),
+        Document(page_content="a", metadata={"source": "1"}),
+        Document(page_content="z", metadata={"source": "1"}),
+    ]
+    assert splitter.split_documents(docs) == expected_output