docs: improve discoverability and docs for the instructions parameter

Add a class docstring to MCPServer documenting the instructions parameter,
and a new Server instructions section to the first-steps guide, with a
concrete example showing how to use instructions to express tool grouping
and workflow guidance.

Closes #1464

Author: QiYuyyds

docs/get-started/first-steps.md

@@ -116,6 +116,22 @@ Notice what isn't there. `completions` (argument autocomplete for resource templ
     `Client(mcp)` is the same in-memory client every example in these docs is tested with, and
     it's how you'll test yours. It gets a whole page: **[Testing](testing.md)**.
 
+## Server instructions
+
+When a client connects, the server sends an `InitializeResult` during the
+handshake. Its `instructions` field is a free-text string that clients can use
+to guide the model on how to use your server's tools — for example, grouping
+related tools or describing a workflow:
+
+```python title="server.py" hl_lines="4"
+--8<-- "docs_src/first_steps/tutorial002.py"
+```
+
+This is the simplest way to express "these tools go together" or "follow this
+order" without building a dedicated grouping API. See the
+[specification](https://modelcontextprotocol.io/specification/2025-06-18/schema#initializeresult-instructions)
+for the wire format.
+
 ## What you did not write
 
 Look back over this page. You wrote three small Python functions. You did **not** write:

docs_src/first_steps/tutorial002.py

@@ -0,0 +1,22 @@
+from mcp.server import MCPServer
+
+mcp = MCPServer(
+    name="Demo",
+    instructions=(
+        "This server exposes two groups of tools: 'read_*' for fetching data "
+        "and 'write_*' for persisting it. Always call a read tool before a "
+        "write tool, and prefer batch_write over repeated single writes."
+    ),
+)
+
+
+@mcp.tool()
+def read_status() -> str:
+    """Read the current system status."""
+    return "ok"
+
+
+@mcp.tool()
+def write_record(data: str) -> str:
+    """Persist a record."""
+    return f"wrote: {data}"

src/mcp/server/mcpserver/server.py

@@ -145,6 +145,16 @@ async def wrap(_: Server[LifespanResultT]) -> AsyncIterator[LifespanResultT]:
 
 
 class MCPServer(Generic[LifespanResultT]):
+    """A more ergonomic interface for MCP servers.
+
+    Exposes tools, resources, and prompts to connected clients, and declares
+    capabilities automatically based on what you register.
+
+    The ``instructions`` parameter returns free-text guidance to the client in
+    the ``InitializeResult`` handshake. Use it to describe tool groupings,
+    workflows, or usage hints for the model without a dedicated grouping API.
+    """
+
     def __init__(
         self,
         name: str | None = None,