> I cannot find any documents that directly relate to firebird.conf
> Where are they?

The conf file documents how it works. It would be pointless and stupid
to create a separate document for a file

# ----------------------------
# External File Paths/Directories
#
# ExternalFileAccess may be None, Full or Restrict. If you choose
# Restrict, provide ';'-separated trees list, where external files
# are stored. Relative paths are treated relative to RootDirectory entry
# (see above). Default value 'None' disables any use of external files
# on your site. To specify access to specific trees, enum all required
# paths (for Win32 this may be something like 'C:\ExternalTables',
# for unix - '/db/extern;/mnt/extern').
#
# NOTE: THE EXTERNAL TABLE ENGINE FEATURE COULD BE USED TO COMPROMISE
# THE SERVER/HOST AS WELL AS DATABASE SECURITY!!
#
# IT IS STRONGLY RECOMMENDED THAT THIS SETTING BE USED TO LIMIT
# EXTERNAL TABLE LOCATIONS!
#
# Type: string (special format)
#
#ExternalFileAccess = None