clonebundles: improve BUNDLESPEC documentation Before the existence of `hg debugbundle --spec`, the process for defining the BUNDLESPEC value in manifests was not very clear and not trivial to automate, especially in the case of stream clone bundles. This patch adds documentation to note the existence of `hg debugbundle --spec`. We drop the reference to stream clone requirements handling because it is now redundant with `hg debugbundle --spec`. While we are here, we further reinforce the importance of defining BUNDLESPEC.