[snmp/agent] Improved loading and unload of MIBs

Improved the documentation of the loading and unloading
of MIBs (plural). also added functions for loading and
unloading a single mib.

OTP-11216
[snmp/agent] Fixed doc broken link to gen_server
Convert some notes.xml files from latin-1 to utf-8

In the master branch, the encoding for most xml files have been
changed from latin-1 to utf-8. The problem is, that the corresponding
files in the maint branch still are encoded in latin-1, and that
a merge from maint to master may bring in characters encoded in latin-1
into a notes.xml file declared to be in utf-8.

To fix the problem once and for all (for the files involved), we'll
need to re-encode the files files utf-8 in maint, and then merge to
master.

Noticed-by: Magnus Henoch
Merge branch 'maint'
Merge branch 'bjorn/xml-encoding-fix/OTP-11310' into maint

* bjorn/xml-encoding-fix/OTP-11310:
  Change encoding of troublesome notes.xml files to utf-8
  Convert some notes.xml files from latin-1 to utf-8
Merge branch 'maint'

* maint:
  Change encoding of troublesome notes.xml files to utf-8
  Convert some notes.xml files from latin-1 to utf-8