Fix documentation of config_indexing

[benoit74]

Current python-libzim python-scraperlib documentation of config_indexing is not aligned with the one of the libzim.

libzim says the method is used to disable the full-text index where python-libzim implies it disables both indexes.

Kinda related to openzim/libzim#1074

I just made a test and full-text index is removed but title index is still here.

[benoit74]

The fact that language is automatically sourced from metadata when available is also unclear ; prevents to know we should config index after configuring metadata

[benoit74]

This is a scraperlib issue, sorry