Hi,
your statement is close to the truth.
sjdbOverhang is the number of bases taken from both sides of the junction and joined together to form an additional "junction" sequence for seed search.
sjdbOverhang =(readLength-1) is ideal to capture a read that has (readLength-1) bases on one side and 1 base on the other. However, since the reads will be sampled by --seedSearchStartLmax bases, even if sjdbOverhang < (readLength-1), the junctions with 1-base overhang will still be found.
On the other hand, if sjdbOverhang is too long, more seeds will become multi-mappers, since the the junction sequences are redundant with the genome. However, STAR transforms the seed coordinates from junction coordinates to genome coordinates, and equivalent seeds are collapsed - so in the end it affects only a marginal population of reads.
More
Some about it in this post.
Cheers
Alex