📝 document timeout handling (#478)

## Description

This small PR adds some documentation on how QCEC handles timeouts.

Resolves #477

## Checklist:

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- [x] The pull request only contains commits that are related to it.
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---------

Signed-off-by: burgholzer <burgholzer@me.com>

Author: burgholzer

docs/source/library/configuration/Execution.rst

@@ -4,3 +4,18 @@ Execution
     .. autoclass:: mqt.qcec::Configuration.Execution
         :members:
         :undoc-members:
+
+Timeout Handling
+----------------
+
+Timeouts in QCEC work by checking an atomic flag in between the application of gates (for DD-based checkers) or rewrite rules (for the ZX-based checkers).
+Unfortunately, this means that an operation needs to be fully applied before a timeout can set in.
+If a certain operation during the equivalence check takes a very long time (e.g., because the DD is becoming very large), the timeout will not be triggered until that operation is finished.
+Thus, it is possible that the timeout is not triggered at the expected time, and it might seem like the timeout is being ignored.
+
+Unfortunately, there is no clean way to kill a thread without letting it finish its computation.
+That's something that could be made possible by switching from multi-threading to multi-processing, but the overhead of processes versus threads is huge on certain platforms and that would not be a good trade-off.
+In addition, more fine-grained abortion checks would significantly decrease the overall performance due to all the branching that would be necessary.
+
+Consequently, timeouts in QCEC are a best-effort feature, and they should not be relied upon to always work as expected.
+From experience, they tend to work reliably well for the ZX-based checkers, but they are less reliable for the DD-based checkers.

noxfile.py

@@ -104,17 +104,18 @@ def docs(session: nox.Session) -> None:
     extra_installs = ["sphinx-autobuild"] if serve else []
     session.install(*BUILD_REQUIREMENTS, *extra_installs)
     session.install("--no-build-isolation", "-ve.[docs]")
+    session.chdir("docs")
 
     if args.builder == "linkcheck":
-        session.run("sphinx-build", "-b", "linkcheck", "docs", "docs/_build/linkcheck", *posargs)
+        session.run("sphinx-build", "-b", "linkcheck", "source", "_build/linkcheck", *posargs)
         return
 
     shared_args = (
         "-n",  # nitpicky mode
         "-T",  # full tracebacks
         f"-b={args.builder}",
-        "docs",
-        f"docs/_build/{args.builder}",
+        "source",
+        f"_build/{args.builder}",
         *posargs,
     )