docs: Add details about OpenAI assistant memory

[OlegIvaniv]

n8n-io/n8n#9406

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[github-actions]

Overall readability score: 43.63 (🟢 +0.01)

File	Readability
n8n-nodes-langchain.openai.md	38.69 (🟢 +9.15)

View detailed metrics

🟢 - Shows an increase in readability
🔴 - Shows a decrease in readability

File	Readability	FRE	GF	ARI	CLI	DCRS
n8n-nodes-langchain.openai.md	38.69	33.34	12.95	17.1	14.85	7.94
	🟢 +9.15	🟢 +1.83	🟢 +1.77	🟢 +1.6	🟢 +0.99	🟢 +0.65

Averages:

	Readability	FRE	GF	ARI	CLI	DCRS
Average	43.63	37.09	12	14.72	13.86	8.61
	🟢 +0.01	🟢 +0	🟢 +0	🟢 +0	🟢 +0	🟢 +0

View metric targets

Metric	Range	Ideal score
Flesch Reading Ease	100 (very easy read) to 0 (extremely difficult read)	60
Gunning Fog	6 (very easy read) to 17 (extremely difficult read)	8 or less
Auto. Read. Index	6 (very easy read) to 14 (extremely difficult read)	8 or less
Coleman Liau Index	6 (very easy read) to 17 (extremely difficult read)	8 or less
Dale-Chall Readability	4.9 (very easy read) to 9.9 (extremely difficult read)	6.9 or less