The help section is always open to improvement and like many of the SE communities will likely change over time.
1) Why open this forum to purchasing questions which lead to manufacturer recommendations? Isn't that a pandora's box?
If you read further down, product recommendations are explicitly off-topic here (and at most SE sites). While product recommendations are off topic there are aspects of purchasing that do not involve product recommendations.
2) PDIOO - Plan, Design, Implement, Operate, and Optimize is a recognized industry lifecycle. If it ain't broke, why fix it?
Referencing PPDIOO (you forgot Prepare) is probably not a bad idea, although this "industry recognized lifecycle" is actually a Cisco process. Not a bad one, and like many Cisco pioneered processes/protocols, I will grant is it widely known within the network world.
3) There are many types of networks - SCADA, cellular, etc. Isn't it necessary to specify IP based networks?
Yes there are, and no it isn't. First, this was never deemed an IP only based site by the community. There was some early talk about expanding to allow telecommuncations questions here at a couple different points.
Second, limiting it to IP specifically would exclude any other protocol that might come along besides IP in networking and any older protocol that may still be in limited use. While short sighted, there are also people who might latch on to IP limitation and argue that as this is an IP site, things like L2 shouldn't be on topic, especially if it was a L2 only network.
Besides, how many network engineers are tasked to deal with things like Zigbee or "white spaces" devices as part of their job?
4) Why "your own network"? What difference does it make if the person wants to know how a generic corporate network works or her own? Why exclude those who do not own a multinode network from asking questions?
The point here is that if you do not own/operate the network you are asking about, you are likely unable to provide the details needed to answer the question. For example, configurations, output of commands, etc.
Additionally, this is a site by professionals and for professionals, similar to Server Fault and as opposed to other SE communities like Super User where anyone can ask anything. This generally allows people to answer making certain assumptions about the audience.
How about something along the lines: "Questions relating to the planning, design, implementation, operation, and optimization of IETF and IEEE-based networks carrying IP traffic are on topic. Questions regarding the configuration of a particular vendors equipment is off-topic."
I personally would drop the "carrying IP traffic" unless that is where the community wants to take it. Also, the last sentence seems to indicate that configuration questions (which make up a major share of on-topic questions) would be off-topic.
If you want to rephrase a certain part of the on/off-topic page (or elsewhere), probably the best way to go about it is to reference the section in the question (possibly along with wny you find it lacking) and post the suggestion as an answer (possibly along with the reasoning for it). This allows other members of the community to post alternative suggestions if they have them and allows the community to vote up/down the ones they feel best represent the community.