Add context to bullet points in 2.1.2 (Extending a PCB)

[nicorusti]

2.1.2: in the bullet point where we have references, we should expand this sentences and add some short context (5/6 words) before the pointer to the other section

  *  For the specification of one PCB, see Section 2.2

  *  For more details on selecting PCBs, see Section 2.3.1

  *  For more details on propagating PCBs, see Section 2.3.2

From discussion with Eliot (28.08.23)