OPG-496: Update docs with template variable syntax

[synqotik]

Template variable interpolation syntax rules have changed somewhat in OPG v9. To reduce ambiguity in parsing, you can only use $variable syntax if the variable is surrounded by whitespace, or at least ends in whitespace.

If the variable is part of a contiguous "word", users need to use the ${variable} syntax. Therefore some constructs that were valid in OPG v8 and before are no longer valid in v9.

Examples - if there is a template variable node:

Valid: $node
Valid (but not preferred): index_$node
Valid (preferred): index_${node}
Invalid: $node_index (this will be an error since OPG interprets this as the template variable node_index)
Valid: ${node}_index

This PR just updates some documentation to explain this syntax.

External References

• JIRA (Issue Tracker): http://issues.opennms.org/browse/OPG-496
• Continuous Integration: CircleCI

[dino2gnt]

The table in your PR comment:

Valid (but not preferred): index_$node
Valid (preferred): index_${node}
Invalid: $node_index (this will be an error since OPG interprets this as the template variable node_index)
Valid: ${node}_index

...would be a great addition to the actual documentation.

[synqotik]

The table in your PR comment:
...would be a great addition to the actual documentation.

good idea, will do!

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[synqotik]

@dino2gnt added a "truth table" :D

[dino2gnt]

Looks great. Thanks!