C docs for xdata functions.

[gaborcsardi]

• [x] Random dot product graphs
• [x] Samplers (sphere surface, volume, Dirichlet)
• [x] Local scan
• [ ] SGM
• [x] HSBM
• [x] Dimension selection
• [x] Correlated games.
• [x] Adjacency spectral embedding

[gaborcsardi]

SGM is not in C currently, so I'll close this.