Clarify conventions

[bambooforest]

Here I'll start a list of things to clarify in the conventions. Note also to deal with:

• missing conventions #8
• add convention clarification: diphthongs #15

To clarify:

• Be explicit about the raised glottal symbol for glottalized sounds (e.g. comes before nasals) by giving an example

[bambooforest]

Pre-nasalized segment conventions are also not mentioned in the conventions page